First back up your db or export to a file before continuing.
I dont know how much you have installed and edited on your site but I would go into myphpAdmin and open your db. find the table xoops_group_permissions check the box and EMPTY the table, then run the following as this is a default from a fresh set up I just ran this morning.
xoops = prefix you set it as when you ran the install if you changed it from the default in xoops_group_permission
-- phpMyAdmin SQL Dump
-- version 2.6.1-rc1
-- http://www.phpmyadmin.net
--
-- Host: localhost
-- Generation Time: Feb 28, 2005 at 11:43 AM
-- Server version: 4.0.22
-- PHP Version: 4.3.10
--
--
-- --------------------------------------------------------
--
-- Table structure for table `xoops_group_permission`
--
--
-- Dumping data for table `xoops_group_permission`
--
INSERT INTO `xoops_group_permission` VALUES (1, 1, 1, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (2, 1, 1, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (3, 2, 1, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (4, 3, 1, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (5, 1, 1, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (6, 1, 2, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (7, 1, 3, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (8, 1, 4, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (9, 1, 5, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (10, 1, 6, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (11, 1, 7, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (12, 1, 8, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (13, 1, 9, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (14, 1, 10, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (15, 1, 11, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (16, 1, 12, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (17, 1, 13, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (18, 1, 14, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (19, 1, 15, 1, 'system_admin');
INSERT INTO `xoops_group_permission` VALUES (20, 1, 1, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (21, 2, 1, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (22, 3, 1, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (23, 1, 2, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (24, 2, 2, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (25, 3, 2, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (26, 1, 3, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (27, 2, 3, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (28, 3, 3, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (29, 1, 4, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (30, 2, 4, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (31, 3, 4, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (32, 1, 5, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (33, 2, 5, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (34, 3, 5, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (35, 1, 6, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (36, 2, 6, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (37, 3, 6, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (38, 1, 7, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (39, 2, 7, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (40, 3, 7, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (41, 1, 8, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (42, 2, 8, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (43, 3, 8, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (44, 1, 9, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (45, 2, 9, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (46, 3, 9, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (47, 1, 10, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (48, 2, 10, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (49, 3, 10, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (50, 1, 11, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (51, 2, 11, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (52, 3, 11, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (53, 1, 12, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (54, 2, 12, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (55, 3, 12, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (56, 1, 2, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (57, 1, 2, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (58, 2, 2, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (59, 1, 3, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (60, 1, 3, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (61, 1, 13, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (62, 1, 14, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (63, 1, 15, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (64, 1, 16, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (65, 2, 3, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (66, 2, 13, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (67, 2, 14, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (68, 2, 15, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (69, 2, 16, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (70, 1, 4, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (71, 1, 4, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (72, 1, 17, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (73, 1, 18, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (74, 1, 19, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (75, 1, 20, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (76, 2, 4, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (77, 2, 17, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (78, 2, 18, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (79, 2, 19, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (80, 2, 20, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (81, 1, 5, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (82, 1, 5, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (83, 1, 21, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (84, 2, 5, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (85, 2, 21, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (86, 1, 6, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (87, 1, 6, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (88, 1, 22, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (89, 1, 23, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (90, 2, 6, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (91, 2, 22, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (92, 2, 23, 1, 'block_read');
INSERT INTO `xoops_group_permission` VALUES (93, 3, 3, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (94, 1, 7, 1, 'module_admin');
INSERT INTO `xoops_group_permission` VALUES (95, 1, 7, 1, 'module_read');
INSERT INTO `xoops_group_permission` VALUES (96, 2, 7, 1, 'module_read');
hope this helps