Deleted

if you dont play a game for a long time it just uninstalls

I had something similar. I had Witcher 2 downloading but then when I paused it for like 2 days it randomly uninstalled.

I've had that problem with HL2, navigate to your steamapps folder and check if the game is still there. If the files are here but Steam thinks the game isn't installed, opting out of the beta might help.

Old games never uninstall itself, just fade away,