Dirt 2 doesn't work anymore

Verify integrity of game cache.

check your display driver comparability

what kind of error is it?

Step 1: Go to the game's Steam forum and look for solutions. If nothing is found, make a thread.

Step 2: Go to the game's Steam discussions area and look for solutions. If nothing is found, make a thread.

Step 3: Contact the relevant support people. They can be found on the Steam support site.

Note: Do not confuse the last step with contacting Steam support themselves. They will only help you with games from Valve. 3rd party games have their own support staff.

Edit: Optional step: Go to the game's official forum and search for solutions. If nothing is found, make a thread. Optional because not every game has a forum.