Peugot 406 remote keys

I have changed battery in my remote key
and now find i cant lock or unlock the doors unless i use the key in the lock.

what have i done wrong? .
and how can i correct it

 

Not familiar with the 406 fob but does it have an LED and does it
illuminate so you know the battery fitment is okay?
Otherwise try reprogramming the key here www.peugeotdiesel.com