Depends on what changes you made to your system since you last played Black Ops. If Steam will let you verify your game cache, then try that and see if it fixes the problem. If it doesn't let you, or if the cache check comes back with all files verified and doesn't download anything in the background (like mine does), try just deleting your Black Ops folder, or moving it somewhere else at the very least, then just install the game through Steam again. If that fixes your problem, then it was just corrupt game files. Otherwise, it may just be a problem with your hardware, or a conflict with something else you are running.
I could help you more if you mentioned things like what else was happening to your PC around the time it stopped working, e.g. blue screen of death, random error messages; basically anything out of the ordinary. Also, any new hardware you may have installed, e.g. new GPU, CPU, HDD or anything like that may be causing your problem.