Grand Cherokee 4.0 petrol 1999 starting problem

[E.S.G]

Hi friend has a problem with the above vehicle. The battery went flat , charged it up. When he went to start it , will not start, it tries to crank but the red immobiliser light on the dash is on constantly, the doors will not lock, although you hear the locking mechanism whirring. The battery was fully charged but we checked it today and it was down a couple of volts. Any idea's ? Thanks