Key Fob not working - batteries flat?

Sorry if this question has been asked many times before... I have 2 key fobs, both were working. The usual one stopped working so I replaced both batteries in both units. Usual ones not working now. The flat batteries were in there for about a month - could it have become un-programmed? cheers....

As far as I know most of the key fobs have unique fixed codes, while the car have to be programmed to accept them.

Its worth reprogramming the key see the owners handbook ! failing that take the fob apart and clean the rubber contact faces on the button and circuit board and try again , use meths or brake cleraner

its unlikely your fob has become unprogrammed, purely because the fob hold a set code which has to be recognised by the on board receiver..... i would first of all make sure the batteries are in securely, the correct type and that the fob makes a blip noise when the button is pressed...if it does then the fob is fine and is likely just to need reprogramming to the receiver using the five ignition turn on/off method...this is dependant you have a fob looking like this of course, the older fobs are a different matter...!! cheers jod