It could have been disconnected previously or if the air bag had been deployed previously and not replaced, it would be on. If the car is under warranty, take it to your dealership and have them check.
Air bag lights are controlled by sensors all over the car which sense when to release the airbag during an accident. one or more of these sensors may have gone bad or shorted out.
There are a few reasons why this could be one I would start with checking all of the fuses in the car that have anything todo with the airbags. Then I would check to see if they are unpluged this should fix your problem.
The light is on because there is an issue with your air bag. There may just be a loose connection or you may need to replace your air bag. You should get this checked right away as it is a safety issue. You probably need to go to the dealer to have this issue checked.
