Just tried to start up my copy and got this error:
'This game needs Steam. Please make sure that Steam Client is running and logged in with a Steam account that has this game in Library, then run the game again. The game will now exit.'

If it happens to you too, do what I did:
1. Verify the integrity of the game files
2. Do a complete restart of Steam. Close it normally, then end the process, and then open it up again, as if you've just turned your computer on

Once that's done it should boot up, at least mine did without issue. Figured I'd make this post in case anyone else has that sudden problem out of nowhere.