What type of fob is it, e.g. lucas 3TXA/3TXB/17TN? The number should be on the back of it.
This is the usual procedure:-
Remove the battery from the fob. Press the unlock button 10 times then press the lock button 10 times.(Apparently this is to drain down the key's memory). Replace the battery.
Make sure all doors, tailgate and bonnet are securely closed.
Lock then unlock drivers door with key but don't open it. Press LOCK button on key fob 4 times (some people say 8 times but 4 seems to work)
Press unlock and everything should be back to normal.
Do you have the E.K.A. (emergency key access) code? If the key won't re-sync this can be used to by-pass the immobiliser.
Brian.J.
July 2008