Miguel, A few things could cause this no reversing to happen. Check the trans. fluid level and condition(does it smell burnt) if it does chances are the clutches are worn/slipping and transmission requires rebuilding. Have checked for trouble codes, if present they may aid in diagnosing. Low/dirty fluid/filter, faulty shift solenoid, bad clutch pack, faulty pump, etc. Your probably best to have checked by a good transmission shop, diagnosed and estimate on repair. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.