I faced a similar situation earlier. Closed all apps, tried battery pulls many times but no luck. Reloaded my OS and it started working, only to stop working after a day. After sometime though all of a sudden the camera started working again!
Try these steps:
1) Close all applications on the bb.
2)Perform a hard reset on the bb.Then attempt to open the camera application again.
3)Backup your bb first. Then Run the bb through the application loader tool.
If nothing happens, there MAY be a physical problem with the camera.
Post to tell us what happens.