As the title says, I cant get any source or goldsource mod to start. When I click play, it just sits there. Also, before this, I was able to play mods. Also, I have all sdk bases installed. Additionally, Steam updated when I started it so that might be related.
Have you tried the usual? Verify/defrag the SDK base caches, reinstall…
I have the same problem, I’ve been playing the game for a couple of days and everything was fine, but today I tried to play the game and nothing happened. I verified the cache of the SDK and downloaded a 9MB update but the problem is still there. I reinstalled the game and the SDK but nothing! 
I finally found a fix for this: Opt out of all betas for Steam. It worked for me after that.
It worked, thanks! 