The simple sync first
BMW fobs use a rolling code; a battery swap can put the rolling counter out of sync. Insert the battery the correct way (CR2032, positive side up on most models), then within ~30 seconds use the metal blade to unlock the door and start the car normally. Drive for a few minutes. The fob should re-pair automatically on most E70 and F15 X5 models.
If sync does not work
If the fob still does not respond, the most common causes are (1) the fob internal board has failed, (2) the CAS module on E-series or FEM/BDC on F-series has lost track of the rolling code, or (3) you actually have a flat second battery. A Master Automotive Locksmith can diagnose with AVDI / Autel IM608 and either reprogram the existing fob or cut and program a new one on-site.
When to skip the dealer and call mobile
BMW dealerships in DFW typically quote $300–$500 for a fob reprogramming and require an appointment 3–7 days out. Mobile locksmith pricing for the same work runs $150–$300 with same-day service. The dealership path makes sense if your X5 is under warranty; otherwise mobile is faster and cheaper.