Computer was working fine. Suddenly won't boot.

Hello forum,

I have a computer that I recently built.

It's been working fine, but it went into sleep mode earlier and then I couldn't get it to come out. So, I powered it down and turned it back on, and now it won't POST. I get no video signal to the monitor.

About a week and a half ago I added in a RAM stick, but it didn't cause any problems during the installation. I suppose I could try removing the stick and see what happens.

A note: This happened to me a few weeks ago, one time. When it didn't boot that time, I unplugged everything from the computer, and then it booted fine. When I plugged everything back in, it stayed fine. I tried that this time as well, but no dice.

Any ideas?

EDIT:

Ok, so I waited about 5 minutes with the computer unplugged, and then I gave it another go. It POSTed fine and I'm back in business. It gave me a message saying that it failed to come out of sleep mode, or something to that effect, and that I could try to use the previous data it had saved related to that sleep state, or that I could just boot windows fresh. I first tried using the previous data and it failed to boot; it started "resuming windows" but then the signal from the monitor went away and the system just sat there. When I tried booting fresh, it was fine.

So this is the second time now then, that my computer has gone into sleep mode (actually, I guess it might be hibernating, I'm not sure.) and then wouldn't POST when I tried to wake it up. Any idea what could be cause of this unstable behavior? There are plenty of times when it goes into sleep / hibernate and it comes out just fine.

Thanks for any insight.