Steam games wil not launch

[t-rex]

I've been playing games on steam for 2 years now and have never encountered a problem. now when i try launching a game the loading screen comes on then goes back off. on task manager i can see that hl2.exe comes on then goes off automatically in a few seconds. plz help ive got a clan match coming up

 

[Beerdude26]

Try deleting your Clientregistry.blob :
Exit Steam. Delete the clientregistry.blob in the main Steam folder and the platform.gcf file in the Steamapps folder. But make a backup of them first by copying them in another folder! Now restart Steam.