Looks like Valve accidentally broke something on their end for source modifications, here's a known fix to help you guys get back in game asap!
1. Close Steam. 2. Delete ClientRegistry.blob in your Steam Directory 3. Run Steam.
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An alternative way to fix the issue if the first method doesn't work:
1. Exit Steam if you haven't already. 2. Delete the config folder in your Steam directory. 3. Run Steam.exe - Steam will redownload all of the files you deleted and verify the contents of your steamapps folder. (Credible sources say this process only takes a couple minutes) 4. Test the game!