I can no longer launch my Steam games. When I hit play it has the pop up saying it is launching then the Steam window blinks for a second then does nothing. It is starting to really annoy me, anyone know how to fix this without reinstalling Steam?
In library view, have you tried right clicking on Portal 2 and verifying game files (or something like that)?
Otherwise, what worked for me was going into the Steam folders in Finder and deleting some of the smaller support files (nothing more than 500MB). Then when you open up Steam it will download the missing files and (hopefully) fix the issue.
Here were the official instructions I received:
"Please exit Steam and go to the folder Users/[username]/Library/Application Support/Steam
Delete all of the files in this folder except for SteamApps.