How can i re-intraduce the sensor in the shed again? ?

I have a veritas 8 alarm system which is protecting my home and my shed at the bottom of the garden. The system is all hard wired. I had to move the shed to get the garden flagged recently for the kids which in turn meant that i had to disconnect the motion sensor in the shed and once the garden was complete i then re-installed it back into the shed. I took great care and disconnected the cables one at a time as not short it out. The problem that i have now is that the sensor in the shed doesnt seem to be working and a red light in zone 3 at the alarm pannel apears now and again. Can anyone help me please? How can i re-interaduce that sensor in the shed again so that it all woks again? Do i need to reset the system somehow? Regards Mr Hughes.