Turned off
Damaged sensor
Damaged sensor wires
Damaged sensor control unit
Unplugged control unit
Blown fuse
Removed live wire
Removed earth wire
Etc etc
Pull off the bumper check all the sensors are wired, this I'd do last as it wouldnt stop it from working it would just make it beep constantly when in reverse
Check the boot for the control unit usually behind one of the panels check the live/earth feeds are connected, chase the live wire to the 12v supply find a fuse check it