If the remote fob is not communicating with the car, the first thing to try is fitting a new battery to the fob (it is a CR2032 button battery).
If you have a black squarish fob with two buttons you will have to press the unlock button on the fob five times in fairly quick succession to resync the fob with the alarm system. If it is a round black and grey fob, there is no need to resync as it should work without.
If the fob battery turns out to not be the problem, it is probable that there is some other fault (on later cars with the round fob, there are known issues with the potential for water ingress via the main loom from the engine bay to the passenger compartment which leaks into the Security Control Unit behind the dash and damages the circuit board for the fob radio signal reciever and prevents the system 'hearing' the disarm code; the security system shares a code with the engine ECU to disarm the immobiliser and there is the possibility that the code has become corrupted (which sometimes happens if the car is left for an extended period with a slowly flattening battery). Damage to the SCU circuitboard will require repair or replacement of the unit, and a corrupted code can be restored by someone with suitable diagnostic equipment.
I will assume you are not in possession of the four digit 'Emergency Key Access' code - this can be entered by turning the key in the drivers door lock (I would advise getting someone with a Rover T4, a T300 or a pscan to read the code from the security system for you once you get it all working again). The entry procedure was changed slightly a couple of times during the production life of the 25/ZR, so it is not really possible to describe without knowing the year of the car at least.
Hope you get it sorted without too much bother - with luck, changing the fob battery may be all that it needs