Hi had the same thing happen with mine, after it had been sitting for 5 years and I then bought it. Jam shut, lock not working, button seized, managed to open it after much distress by undoing the top hinges and with a bit of wiggling and pulling side to side managed to get it off the locks. (wasn't easy).
On the inside lower edge of the upper tailgate there's a cover piece attached. If you can remove this, or bend it away (it's only thin steel) you can expose the rods on the latches (there's one each side). Get pliers/grips on and a swift tug can release the catches and get the tailgate open.
On mine, I left the cover off in case the lock jams a second time