could be a broken wire, a bad lift gate module, a bad lock actuator, or the door may not have completely latched due to an obstruction
does the rear wiper work?
does key fob work?
can you open the glass with all doors unlocked? how about locked?
the rear lift gate has a computer module in it that controls the functions of that door, it also acts as the receiver for the fob remotes
if it is the lock - you can manually unlock, on the inside of the hatch, near the center bottom, there should be a round plug/cap
pry it off to expose the locking rod and with a finger move it which ever way it will move - this does not require much force
then climb out and try the handle