At the moment mine is fine, but I have experienced this before. It just came back after a while. I don't know what caused it, I presume some sort of maintenance was being performed.
It came back during the afternoon. A Cpanel update did not upgrade properly on the weekend, so I was told.
As usual, those kind of things keep hapening to me. The only thing I needed to access during the week-end was Fantastico, and it was the only thing not accessible. I have always been lucky !
There was an update to Cpanel which has a new security feature that made it imcompatible with Fantastico. There was a workaround posted by the Fantastico developer which has resolved this issue.