iMac Won't Wake from Sleep: I Have to Restart

I bought my iMac a couple years ago. I put it to sleep each night. I recently upgraded to Snow Leopard. Not sure why, but for the past week I can't wake from sleep mode. I have to hold the button in back to restart. Is it Snow Leopard? Is my machine dying? For the time being, I will hold off on putting it to sleep and just run the screensaver. Any ideas, though?

Thanks...

While i have not made the step to SL - couple of steps i would try - firstly download and run the combo updater for latest SL - often the combo updater will sort minor issues and then if no better you could try resetting your SMC - this link here for my unit

I should point out that everything is super slow now, too. When I installed Snow Leopard, it seemed faster... for a few days. Now it takes 30 seconds to load the dashboard. I get the beachball now and then when trying to load a page or two. I've got about 252GB free (212 used). Is it a process? I don't know if Time Machine has anything to do with it, but it keeps trying to back up all the time. Put it in sleep mode, again, last night and had to restart, again.

I don't know. I followed the directions and unplugged everything. We'll see if that helps. I don't have many apps open. Firefox, Newsfire, iTunes and Lala.

I deleted a few apps that I've installed recently (example. Camtasia). I had iTunes crash on me tonight (it's done it a few times since the new version launched). Firefox crashes now and then when playing flash content, too. It's still really slow, though. I don't know what it can be...

Quick update for those who may stumble across this in the future...

I unplugged everything, then booted up. Sleep mode is fine now.

What about your other problems you were experiencing? Are they all cleared up also since you unplugged and restarted?