I occasionally have this problem if I am switching between multiple Steam accounts. I don't know what the actual reason is, but it wouldn't hurt to verify the game cache integrity through properties. I would try that and see if it helps.
DJ, i had the exact same issue. Id be willing to bet that you can't view any of your replay's either (if you care about that sort of thing). I solved this issue by simply uninstalling and then reinstating tf2. fixed all my issues. I would recommend to save a copy of all your scripts if you have any for your classes.
i had the same problem, and when it finish and go to the game, after few minutes it crash, i reinstalled steam twice, same with the game donwloaded twice and i still have the same problem, any advice???
I remembered I had this problem once, which lasted a while and worried me before. I did not know what to do so I reinstall my TF2. I hope you do not have to do that.