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Do you have access to your MySQL database? You should only do this if you are familiar with MySQL. Otherwise you should get someone else to help you with it.
If you have access and feel comfortable with MySQL, go into phpMyAdmin, eskuel, or your favorite MySQL manager. Open your database and find the xoops_config table (with your table prefix, of course, in place of xoops_).
There, run this query, replacing xoops_config with your table name:
UPDATE xoops_config
SET conf_value=0
WHERE conf_name = "closesite";
That will reset the variable to open your site back up again.