If the hatch latch or its linkage is frozen, the lift gate trim panel will have to be removed from the inside (at least at the bottom half) so you can get at the latch area. You may be able to see what is wrong, apply lubricant and manually unlatch it.
If the lift gate hasn't been used regularly, they can rust up.
Getting the trim panel off from the inside with the lift gate closed is more challenging than with it open, but can be done. Plastic is more pliant when warm. See p. 23-183 here: http://oskin.ru/pub/chrysler-dodge/...01_RG_Town&Country_Caravan_Voyager/01RGEE.PDF
Figure 2 shows the latch and linkage.