Protected Page Not Working

Have you tried:

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the Twenty Eleven theme to rule out any theme-specific problems.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

Can we see the site using the Twenty Eleven theme with all plugins deactivated? Also, can you reset the password and let us know what the new password is.

Can you try resetting your custom permalinks back to the default setting and renaming the .htaccess file in your root folder?