Rear door won't open - 1999 Land Rover Discovery

The rear handle gets stuck in the up position..spray with wd40 and work till released..then it will be fine

If by Rear door you mean the boot door, then pry the land rover badge from the plastic and use some WD40 or similar to lubricate the handle (which you can push back down to gain access with).
If however, you mean the rear side doors, then this is more likely to be the door lock unit, and this can be caused by the actuators in the lock mechanism failing usually causing the door to remain locked. The boor door can also fail this way, as do the front side doors. Replacement requires the whole lock unit to be replaced at a cost of roughly £60 a lock, and a hours work. Helps if you have small hands.