Members can't login to my site

Yesterday this problem started.  Some of my members started to email me saying that after they enter their email address and password and proceed to login, all that happens is they get a white screen and cannot view the website again until they clear their cache.

After this started happening, I tried to login with my main "superadmin" account and the same thing.

I'm wondering if it has anything to do with the "remember me" option being selected on the login page...

Any suggestions welcome - i'm worried this is going to start happening with all my members accounts which would be horrible....

Thanks!