If you've been doing things to the game folder this could also cause you problems launching a game if you don't know what you're doing.Playing games for the first time can also have this problem.
A common reason why this can happen with Rocket League and other games is due to an update not installing properly. Trying to fix the Rocket League won't launch problem can be tricky since there's a range of potential causes for this. It's always annoying when a game suddenly decides to no longer launch and Rocket League is no different.
