do not despair! I am pretty sure this is a random thing that happens, I remember it happening to me once. I rebooted my system and it worked, but I think there's something else you can do if that doesn't work. There is a file you can delete in your steam folder that will reset somethingerother, and allow this problem to go away. I wish I could remember it, but I do not. Someone else surely knows what it is, and I'm only posting to let you know that you won't need to buy another copy of half-life.
so whoever knows what that file is, do tell.