Replaced a rear door handle now door will not open

It sounds like the door lock may have tripped and bound up and it's preventing the handles from moving the latch release. With neither handle working you will likely need to remove the inner door panel to get at the latch and manually trip it inside the door. Before that here are a couple things you might try. Push in on the door and hit the unlock button a few times then have someone inside the car pull the interior handle and push out on the door, then release the pressure on the outside of the door. If you are lucky the combined forces will allow the lock to release and the door will open. If that doesn't work try using a rubber hammer inside near the latch as you hit the button. That might jar it loose.
Removing the door panel isn't difficult if there is room to pop the retainers free, the hard part will be getting between the panel and the door with the door closed. In these cases I normally remove the rear seat to get more room, then remove the panel. You have two screws and 7 clips to get the panel free. It will pull free of the clips and lift up to get free of the door. If it has power windows disconnect the switch. With the panel off use a tool to gently remove the inner weather panel. That should let you get to the latch and the locking solenoid. The part near the electrical connector is the locking piece, one direction locks the door by disconnecting the opening lever from the latch, you should be able to reach in and move it enough it releases when you pull the handle.