First check that one of the sensors has not become detached inside the rear bumper.

When this happens, the sensor "sees" the inside of the bumper right up aginst its figurative nose as soon as reverse is engaged. Many reversing sensor systems assume that this is an error[1] and disable themselves to avoid filling the car with a continuous, raucous beeping.

[1] Engaging reverse while the back end of the vehicle is already right against a wall not being common practice.

Thanks elekie&a/c doctor.

I'll take a look at that over te weekend.