Games : My Steam gives an "Failure to start game Unknown Error" on games that are already installed and I've played it earlier in the day

Earlier today, I played Sonic and All Stars Racing Transformed multiple times between 8:00 AM and 2:00 PM. At around 3-ish it gave me the failure to start game error. I tried all the other games and they all had the same error. All of these games have been played before and are installed on the computer. The internet was working and Steam was online, as I had a notification pop up that one of my friends was playing TF2