Imac keeps restarting and turning off on it's own.

Question

Just recently my iMac 21.5" 2010 (10.7.4) has been restating and turning off unexpectedly on it own. I've had this Mac for over a year and have never had this problem before and haven't even changed any settings or done anything to it.
The biggest thing I've done to it was update it's software.

Answer 1

Hello, have you blown the dust ou tlatey?
If it's cooled foran hour or so dies it Boot?
Do you have the extended 3 year AppleCare?
"Try Disk Utility
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
*Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair Disk, (not Repair Permissions). Disk Utility checks and repairs the disk."
http://docs.info.apple.com/article.html?artnum=106214
Then try a Safe Boot, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, reboot when it completes.
(Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive.)
If perchance you can't find your install Disc, at least try it from the Safe Boot part onward.
If 10.7.0 or later...
Bootup holding CMD+r, or the Option/alt key to boot from the Restore partitiion & use Disk Utility from there.