How can i reset my black box car alarm?

I disconnected the battery on my car before i went on holidays. When I came back and connected the battery the car wouldnt start because the alarm is on. I have changed the battery in the key fob, thinking that was the problem, but it still isnt turning off the alarm.
I think that by disconnecting the battery it has lost the code or signal to the key fob.
Any ideas how to resolve the problem?