I have this problem occasionally, and I always find turning my PS3 off via the button first, then by the plug socket for a bit helps. However, if you have a 'fat' PS3, turn off the switch at the back. I have a lot of games downloaded from the PS Store to my HDD and since I've had these, my PS3 has been slower at times. Try taking the disc(s) out, turning the PS3 off, turning it back on and retrying. It may also be worth disconnecting the PS3 from the internet completely (not just signing out of PSN) and trying to load games.
I hope this helps!