2008 iMac won't turn on after Mountain Lion installation

I have a 2008 iMac with 6GB of RAM that was running on OS X 10.6.8 perfectly. Tonight I upgraded it to Mountain Lion. After the installation, my machine was running slower and a little choppy, especially when first opening programs, even though it meets all the requirements for Mountain Lion. After about an hour or two of running like this, I was standing away from the computer for a minute when I saw a flash come from the monitor. I went to it and the screen was black. I thought maybe it had went to sleep, but nope! It was completely off and would not turn back on. No lights or sounds come on at all. There was no power outage or anything, and all other devices plugged into the same surge protector were working fine. I tried plugging it into another outlet and still had no luck.
Any insight onto what caused this? Is it most likely a power supply failure, and could this have been caused by the Mountain Lion installation? Considering I've had this computer for almost 5 years and this has never happened before, it seems obvious that this contributed somehow.
Any advice would be appreciated. I'm going to try to take it to an Apple store soon, but the earliest appointment is a few days away.
Thank you!

Welcome to Apple Support Communities
It's quite possible that the power supply failed. The failure is difficult to be related with OS X Mountain Lion, so I would discard that. In some cases, it's normal that you experience slowness after installing a new OS X version because Spotlight indexes the hard drive.
The best you can do is to take it to an Apple Store or reseller so you can get your Mac diagnosed. Now we can't do anything else apart from thinking that is a power supply issue

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http://support.apple.com/kb/TS2802
Will not turn on, will not turn on unless connected to power, or unexpected power off
Verify that the Sleep/Wake button functions. If it does not function, inspect it for signs of damage. If the button is damaged or is not functioning when pressed, seek service.
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