Yesterday I was playing GTA5 Steam Version just fine, today the "Grand Theft Auto V" font-color in my Steam Library isn't white anymore just greyed out, also the Installation button returned, all fine and good I thought, I had this Problem before, Steam would just find the existing files and I'm good to go but after the loading bar reaches the end it displays "Steam is now downloading GTA 5" which pretty much sucks, I mean it's 60GB come on...
Anybody else had this problem with another game perhaps?
SteamApps/Common
folder to see if GTA is still there, if it is, make sure Steam is downloading onto that partition, in case you have multiple.