how do i get sigma m800 to recognise the original transponder?

2yrs ago after changing the batteries on my truck the immobilizer somehow tuned itself to my transit van key and no matter what I do I can't get it to tune back to the original transponders. The transit has gone now but up until 2 days ago I still had the key that worked,my wife lost the keys and now the truck is stuck on the drive. I have the spare keys and the original fobs but cannot get it to work, can anyone help?