No reverse in transmission

Reverse just quit out of the blue.
All forward gears are fine.
You can push vehicle backwards in neutral or reverse.
Fluid levels are fine, no fluid in radiator.
No noise coming from transmission.
I've checked codes already and have 1 knock sensor and 1 knock sensor pending I erased them and checked them again, no change. Am going to change fluid and filter, but couldn't the inhibitor switch as well as the speed sensor be out? As well as a relay or fuse? And what about the gearbox reverse switch?
Am praying it's not internal. Need all the advise I can get, thanks.