For the last 2 days, many people have been having: Disconnect: Invalid Steam userID or Invalid Steam ID Ticket. I tried everything, including deleting blobs, restarting steam, restarting my PC; and restarting my router. For some reason my laptop worked fine. So I was searching the steam forums, and it seems I am not the only one with this problem. For the last 48 hours HL2/OB mods have not been working for a ton of people, but someone recently posted a fix, which works:
Go to your steam folder, and delete everything except for Steam Apps and Steam.exe. Steam will be prompted to download all the steam files (not your game files) again after you try to launch it.
Topic with the fix: