1) Have you checked and charged the car battery - that is the first port of call if the car won't lock / unlock.
2) Are they the right fob batteries - they need to be CR1620 Coin cells (not CR2032).
3) Did you resynchronise the keys correctly?:-
- Get in car, close door
- Put fob one into ignition and turn ignition on
- within 10 seconds press one of the fob buttons for a few seconds
- Turn off ignition and remove key
- Repeat for other fob
- get out of car and try locking / unlocking