WoW uninstalled itself

I feel alseep to watching a movie on my computer last night. When I woke up the computer had rebooted itself as all applications that were up had shut down. The weirdest thing about it though was when I opend the Battle net app and saw that WoW needed to be installed. I searched my computer but could not find any WoW related folders.

Why did this happen?

I asume your pc crashed somehow, maybe with a bluescreen and the drive that also has WoW on it got corrupted.
You should check the windows event manager for a crash log and also scan your drive for errors.