Get a professional locksmith out to do it. Removing a broken key is usually quite simple if you have the correct tools.
If you start messing about with it you'll probably make things worse and more expensive to fix.
People often try putting superglue on the bit of key that has come out and pushing it in to the lock, hoping it will stick to the bit that is stuck, but they end up with both bits stuck in the lock and the mechanism jammed too!!
Please, call a reputable locksmith, it will be cheaper in the long run.
genifer
June 2011