Cannot Log in to Control Panel

Although the site still shows up one of the WordPress sites I work on is no longer letting anybody login to the control panel. This was happening intermittently yesterday but clearing cookies worked. That is no longer working. If I try to log in to the panel I get a 500 error.
There were two pictures on this site that had malware warnings because they linked to a site flagged for malware. Today I was in the process of taking them out when the problem occurred. Any ideas please?