problem with sleep

I never used to have this problem but lately when I try to wake the computer from sleep it doesn't respond, usually I can hit a few buttons on the keyboard or move the mouse around and I would get the password prompt, Now I have to actually unplug the computer and restart it in order to get it working again. Anyone know what the problem could be.

iMac
Processor: 2.0Ghz intel core 2 duo
memory: 2GB 667 Mhz DDR2 SDRAM
OS X Tiger 10.4.11

garsh....no idea. My only "sleeping problem" is if I sign out, I have to click and wait 3 times for the sleep button to work.