I have same problem... I bought few days ago Arma 3 key, then I activated through steam and play it... No problems. Today morning I wanted to look in my steam inventory if I have Arma 3 aplha lite invites for my friends and nothing was there. So I thought that I must start Arma 3 and then look to my inventory, but in my library wasn't Arma 3. I look in my folder where I have installed my arma 3 and in this folder arma 3 is there. I tried to start arma 3 through arma3.exe and just main page of arma 3 in steam appeared. Steam just acting like I never bought this game. I've played 3,6 hours. Last time it was in Wednesday. My steam account is Kocy24 alias Cookie Monster. I'm from Czech republic and in steam I'm located to Czech republic. I don't know what's going on I've been waiting for arma 3 long time and when I finally bought it... It just Disappeared.
Please if someone knows what to do help me.
PS: sorry for my english