There's a nasty Half-Life 2 bug out there that is stopping people from playing this seminal shooter. Valve's Erik Johnson sent out this to help those folks:
For people that are running into the bug on startup that looks like:
"Map Version is 0, expecting 19"
Try this to see if it works:
1. Go into your SteamSteamApps folder.
2. Rename the file called sourcemodels.gcf to something else.
3. Relaunch the game.
This is going to require a redownload of this file, but if you want to get playing tonight it's probably the best path. There are a couple of people now that were in this situation that were fixed this way.
As soon as we possible we'll release a fix for this that will not require this workaround.
Please let me know if this does or does not fix this specific problem for you. Just shoot me an e-mail at firstname.lastname@example.org. If this does fix it, it would be useful to get copies of a couple of the renamed (bad) cache files.