I hooked up my PS3 to a sdtv, but it's just a blank black screen?

May be...

I can hear the game, but I can't see it.

dude. hold the POWER button for 10 sec while its on then it will restart. BOOM its working now :P

Turn off your PS3. When you turn it back on hold the button for 5 seconds until you hear it beep again... it will reset your ps3 settings so you can see it again.