I have to agree... that crack you're seeing is a red herring. You could probably remove that entire shroud, and the lock would still function fine.

A few things to make a lock not work right:

• Key not inserted properly. All the way in?
• Pressure on the bolt part of the lock preventing from turning the tumbler. Make sure that has at least a little play.
• Something jammed with the lock mechanism. Was the key in the lock when it was dropped?

I'd probably pull the key out, squirt some WD40 inside the tumblers, as well as around the bolt part of the locki, and wiggle everything around a bit.