You activated a buggy PHP-Nuke module. Now, your site is broken and you get a blank page, or you only see errors and the administration panel is
unreachable...
Connect to the MySQL database and change the value of the "active" field in the $prefix_modules table to 0 for that module's entry. Example (see Site crashed when block was created and logout-Fehler): suppose the broken module is Mod_Broken, your $prefix is "nuke" and the value of the module id, "mid", field for the Mod_Broken module is "7"):
UPDATE nuke_modules SET active = '0' WHERE mid = '7';
or (if you prefer to select by title and not by module id)
UPDATE nuke_modules SET active = '0' WHERE title = 'Mod_Broken';