Computer Won't Wake Up From Sleep Mode


Several times over the past few weeks, my computer has gone into sleep mode (dark screen) and when I press keys to wake it up, it won't. The screen remains blank. I had to restart the computer the first time. The second time it happened I couldn't turn off the computer by pushing the power button because it wouldn't respond. I had to unplug the computer to get it started again. Do you have any suggestions as to what to do to fix the problem?

I use an iMac G4 with OS 10.4.2.

Thanks for your help.