Let's see if your camera isn't 'broken' first. Here are a few things you could try:
1) Go into the Application Manager, "All" tab and find the Camera app. Now there, clear Cache. Check your camera again. If not working, try the same but this time clear Data.
2) Not working yet? Try downloading a third party camera app (any free one will do). Does the camera there start or does it give you the same error?
If number 1 fails but 2 doesn't, the problem is your Camera app. Unfortunately, other than a factory reset or manually re-installing the APK from somewhere, I don't know how to restore this one... But if number 2 also fails, then I'm afraid there's something wrong with your camera overall. Might be a loose connector, broken hardware or just faulty software driver (for the latter, a factory reset should clear the issue).