Question : Problem: iMac will not wake up after going to sleep

This is my mother's iMac (I'm visiting). She says that about a week ago she started having a problem that after she puts the iMac to sleep, it won't wake up. I just witnessed it. The "sleep" light is blinking, but if you hit any key to get it out of sleep (like you're supposed to), nothing happens; screen stays black. You can't force a power off by holding in the power button. The only way to "fix" it is to unplug the computer, wait, plug it back in. Then when you turn it back on, sometimes it comes on and sometimes it doesn't. You have to play around with unplugging/turning on to finally get it to work.
Max OS X vsn 10.4.11
I'm thinking it might be software related?
My  mother says it might have started happening after she last updated, but she's not sure.

Answer : Problem: iMac will not wake up after going to sleep

That's probably a G4.  I think the second link that I sent outlines how to reset the PMU in this version.

However, I do think that it's odd about the time thing.  I can't imagine anything that her ISP could be doing that would make that happen.  

Does she have any power issues in her area.  I know that I'm stretching here, but that might whack the Mac.  If she does, a UPS might be in order.