This happened to me once. I just uninstalled Steam and CS:S (When it happened to me it was CS:S) It shouldn't be any different for you. Just uninstall it and reinstall it. If that doesn't work pm me I'll try to help you further.
Also this was in wrong section. Should've been in the Tech Support section. Just for future references think about what you're posting before you post it so you don't end up putting it in the wrong forum. No worries though.
Give me a pm or post here if you need any further help.