[ubuntu] Weirdness...Ubuntu was wiped off my system!

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http://ubuntuforums.org – Last night when I shut my computer down, things seemed fine. But this morning was a disaster...Ubuntu was wiped from my computer. All I got was a message about Grub recovery on a black screen, with a cursor to type commands. I had to reinstall Ubuntu 12.10 all over again. It was a real inconvenience. My question is how this could happen??? I thought it wasn't possible for Linux to get viruses, yet this behaviour certainly sounds like a virus corrupted things! I added a few repositories last night, but that was all I did! Could that have done it? I'm so confused... (Hardware)