That's usually caused by alt-tabbing out and going back in, just reload your HUD.
Thank you for this, if only I actually liked this game so that I would have to use your information.
Oh, I forgot to mention one thing: when first starting TF2, you get stuck in this crappy loadup process which really doesn't say anything useful and is just a pain in the ass. Next, I think we can all agree how the server load screen is garbage.