Try what Weaku suggested, and if that doesn't work, then (using an AVI connection), back up everything you can from the hard drive, and go to System Settings > Factory Reset.
It'll wipe your hard drive, but might fix any glitches that have wormed their way into the system. Worked for me when it stopped launching any games (it could still recognise the type of disc, and even play the XMB animation on PS3 games, but trying to actually play a disc would just give a black screen).
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"I'm Commander Shepard, and you're standing in the biggest library in Citadel Space. Look me up."