Are you VAC banned?
If not, try to log out of steam (change user) and log in again.
Also i read that going offline on the friends list might also help in some cases.
I'd also recomend that you disable automatic login for your steam account and rather do it manually every time.
Also it might be a good idea to disable the automatic login for the friends list as well.
If it still does not work, check if you're able to play any other multiplayer game using VAC.