I didn't play Raceroom for a few month and today I noticed that it is grayed out in my steam favorites and so I tried to launch it and the install-window from steam appeared. The game data is still in the right folder but steam doesn't seem to "know" that. How could that happen?
Maybe the Steam file that holds the information about your game installation was corrupted. If the game files are still there, Steam should automatically recognize them during the installation and not download them again.