Check your internet connection. maybe your ISP having problems accessing other sites. I have the same experience before.
EDIT: Back up in just a few minutes <3 google Server Error We're sorry, but Gmail is temporarily unavailable. We're currently working to fix the problem -- please try logging in to your account in a few minutes.
one day ... gmail was down for almost 4 hours ... it was over night though ... so not too much traffic caught it ...