Do other games work ok?
And have you tried just starting the PS3, signing into your account, then leaving it to sit for a few minutes?
if the same thing happens, I had similar a little while ago - had to send my PS3 off for repairs. It sounds suspiciously like the problem I was having. Also, does the light stay green on the console, or switch to red or yellow? (if it turns red and starts flashing, it's definitely the problem I had, and yellow is a sure sign of problems)