Truck (Isuzu npr 4.8 manual) went for its MoT with every gear working - passed test - when came back (5miles drive) we couldn't reverse it into yard. Just wouldn't select reverse. Checked the two cables, they seem ok and select all other gears no problem.

First check there is oil in the gearbox at the right level
second pull back the rubber boot and look for foreign matter interfering with the gear changer or cabling ie twigs branches and dont laugh rodents! ie those who seek out warm places like exhaust pipes

liberally spray some grease or oil over the linkage under the gearchange boot and see if was just dry metal on metal