Hi Sarah. Try this which worked for me when my fob suddenly stopped working on my Puma. Mine is a three-button fob. For a two-button fob run a google search.
The ignition switch has four positions: 1 off, 11 Radio etc, 111, Ignition (dash light on) and 1V Start.
Sit in the car with doors shut. Put the key in the ignition switch and turn to position 11. Then switch from position 11 to position 111 quickly several times until you hear a 'beep' from the dash (some dash lights may flash). Turn the key off and quickly take it out of the lock and point it towards the dash and press and hold any of the buttons until you hear another 'beep' from the car (telling you the car has detected the key and established a link with it).
Your central locking should now work unless something else is broken. Try this a few times at least.
Good luck
Phil
August 2009