So it did play Black Ops at first but has since stopped, but it does still play other games, right? Are the other games installed on the hard drive or are they on disc? If the PS3 is still reading other discs and not Black Ops, I'd probably assume it's a bad disc rather than a bad disc reader, so see if you can borrow a copy from a friend or rent one or something. If that doesn't work then the next step would be to clean the disc drive laser, a can of compressed air is probably not going to do any harm but it's not the best method for cleaning the laser either, I don't think, but I'll let you Google the best method because I haven't cleaned a laser in years. The things you've heard about compressed air probably relate to flushing dust out of the cooling system (fan, heat sink and vents).
If cleaning the laser doesn't work, maybe you could get a replacement disc drive, failing that then the PS3 is probably buggered to the extent that it's just easier and/or cheaper to get a new one. However I think you can pay ridiculous amounts of money for the latest Call of Duty as a download on PSN which would mean you can still play something on there with a faulty disc drive, so I guess that's something.
