I had this problem a few Christmases ago with my Ford Focus. I'd had the car a couple of years by then so it wasn't new, but all of a sudden the key fob wouldn't unlock the car no matter how many times I tried. I walked into town to buy a new battery for it and it still wouldn't unlock the car.
In the end I simply unlocked the car old school by using the key in the lock. This seemed to reset the locking system when I opened the drivers door. It took a few pushes of the key fob to shut the alarm off, but all was well after that. Weird.