HELP! Portal 2 has disappeared.

[PsychQ]

I used to play portal 2 about a year ago but have had too much school work to do anything on steam. So I came back today to play portal 2 and it's completely gone. I only have 1 account and when I right click the steam icon on the bottom right of my screen it still has Portal 2. When I click that It launches the game but then crashes after the screen with p-body and Atlas.
I can also press shift+tab and it comes up with the normal dashboard and says gametime 46 Hours. Also if I try to activate the serial number again it says that the code is already assigned with the account RAGERanger101 MY ACCOUNT! Please help.

[PinkGIow]

Gabe has killed your portal D:

[PsychQ]

:(