[ubuntu] Sudden shut down

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http://ubuntuforums.org – I have problem with my computer shutting down unexpectedly. I was transferring a text file from a CD to my HD and suddenly everything crashed. I had Firefox and Libre Office running and both programs closed. when the programs closed, the monitor flickered, then all the desktop and Unity icons disappeared. The only thing showing was the photo I use as a screensaver. I rebooted and everything came back, but this happened 3 times today. It's kinda weird. Anybody have a similar thing or fix? Thanks Monitor: 24" ASUS 1080p; Processor: AMD Phenom I quad; HDD 640 Gig; 4 Gig Ram; Sony CD/DVD (Hardware)