How it happened? Well I see the first two replies covered that, There are in factr 3 ways it can get bent down
1: You hit something
2: You backed up
3: Vandals (doubtful as they don't like to work that hard)
NOTE there is a 2B: Towing WITHOUT an aux brake ssytem on the towed and you had to stop ... Fast.... A good aux braking system can lessen the stress on the base plate when that happens... It also helps prevent the "Toad passed me at the stop light" syndrom someone else mentioned.
How to fix it: Two answers,
1: Recommended: Replace.. that is how I'd do it.. REPLACE
2: My fear is this would weaken the base plate.. A good body man can straighten it, NOT the modern "Collix" type replace the part places (They would only replace it anyway) but some old codger in this 50-60-70's who's been running Bump and Dent shops since he was still .. Well he started in his teens as an employee... But again I stress my fear this would weaken the base plate and do not actually recommend save as a very short term solution.
But do check yhour brakes.. A good aux brake can seriously reduse stress on that device.