2000 Chevy Silverado one ton truck. This truck has an automatic transmission 4L-80 E My son was using it to back and forth to school then one day no reverse
Maybe a bad vehicle speed sensor on the transmission. If the control module senses a speed reference greater than 8 mph during the shift into reverse, it has the ability to prevent the shift from happening. Try disconnecting and see if you have reverse.
Otherwise probably will require a pressure test by a transmission shop to diagnose.