Hammer wont launch (gameinfo.txt is missing)

[Capp]

Suddently, when trying to start hammer, it refuses to open and shows the message: Team fortress 2\hl2\gameinfo.txt is missing.

I checked and it really is missing. What exactly am i supposed to do so i can fix this? Please answer.

 

[theatreTECHIE]

There should be an option box when you launch hammer asking what game to launch. Choose TF2, as if you choose half-life you'll get that error.