Suddenly yesterday I was unable to log into the back end of my site and as far as I am aware I have done nothing to the site since I last logged in. It appears that log in credentials are not being checked as I get the log in box but no matter what you put in for user name and password (correct or incorrect) there is only a blank page displayed.

I have another site on the same server and that is working ok.

I have restored from a previous back up but still have the same problem and have no idea where to go now. I have reset the admin password through phpMyAdmin but this did not help either.