You need to get or fashion a large plastic or wooden wedge, about 6-9 inches long and about 3 inches wide..
You then need to place the wedge in the door at the top and prize open the door a little. Then, if you can find some, get a long steel or stiff plastic rod that you can make hook at one end; using the hooked rod, go in and down through the gap in the door and feel for the door latch. It might be a bit iffy to get at, but it does work.
Best of luck.
Dr Antony