You all of the sudden cannot login to Interspire Email Marketer.

You are entering the correct password and the login screen refreshes when you enter the correct login details but will not allow access to the admin panel.

Your problem is the session cannot be saved.

Check your php.ini file to ensure the location for sessions is writable/exists.

Also check /var/lib/php/sessions (CentOS)

Once your sessions can be written to disk, you will be able to login.