Can you no longer select reverse? Sometimes releasing the clutch slightly before trying to shift into reverse will stir up the gears enough to allow selection. Another technique is to select first gear to align the gears before trying reverse.
If this isn't the problem, something may have broken inside to gearbox, or the selectors are faulty.
If this isn't the problem, something may have broken inside to gearbox, or the selectors are faulty.