BrainyBrony

I've left each game for a while, no luck. I'll try ending hl2.exe now.

Nope. L4D2 refuses to start up.

Won't start at all, or still just does the crashy thing?

Try going to your Steam Library, right clicking L4D2 and going to "Verify Integrity" or whatever it's called.

If worst comes to worst, you can re-install the game :/