BMW x5 3.0i sport E53 rear sensor problem!

the PDC on our car was fine only once, one of the rear sensors popped out but we pushed it back in and it was fine! only now when you put it in reverse, it just makes a constant steady beeping sound. it would cut out for a second then do it again. Is it because of the case of the sensor that popped out because the lock on it is damage or is it something else.HELP!!!!