2
Apart from the usual -
check here, try accessing www.yoursite.com/admin.php - if your login hasn't timed out, your should be able to get to your admin area and fix things.
Option 2 - from cpanel or FTP, rename your theme folder to 'badtheme' or something similar, then rename the default theme folder to match your theme's original folder name. Clear templates_c, except for index.html, and off you go.
Option 3 - using phpmyadmin, find the table (xoopsprefix_config) that stores your sites preferences and edit the selected theme (theme_set) to be 'default' instead of 'yourtheme'
Do backups, yada yada, and always have a second browser window open with your site loaded in it when you are doing stuff like this (says so in the FAQ comments, too)