- Corrupted files
- Wrong CHMOD
- Overwritten includes/DEF_ files with empty ones
- Corrupted THEMES table
- Corrupted Cache
First of all make a full site backup, and then you can try one or more of the following:
- Reuploading all files (pay attention to not replace anything you may need and always make a backup first)
- Replacing includes/DEF_ files with fresh ones
- Applying correct CHMOD permissions
- Synchronizing FORUMS
- Uninstalling all styles and install them from scratch (empty themes table and manually add Ice style)
- Empty the CACHE folder (delete all files but .htaccess and index.html)
- Optimizing and Repairing DB (usually via phpMyAdmin)
Try to have a look at this as well: http://www.icyphoenix.com/viewtopic.php?t=1520
There is only one problem... if is the DEF_AUTH being corrupted... then the only solution is to reset the AUTH table. This can happen when upgrading from some old phpBB version with corrupted AUTH table or with some incorrectly deleted users.