I'm actually experiencing the exact same issue, it also began on Friday for me. I found this post while looking for solutions and have come up short.
I tested the game at my office and found that it works normally there, even after installing the most recent version of Steam. Possibly something connected between Steam's most recent update > Our computers/Network? (I recently built this rig to run Black Ops 2 about a month ago, it ran like a charm just the night before)