[Neil]

If it is a turn to open type handle you will have to try and push the latch back into the door. You could try using 2 small screwdriver or knives. push it back a bit with 1 then hold it with the other while you reposition the 1st to push it back some more.
I know it sounds like an old wives tale but you might get it with a credit card. tesco saving card or whatever. You have to try and get it behind the curved part of the latch and pull towards you to push the latch back into the door.

Their is another daft thing it could be, try removing the handle, I have seen it where the square bar has been cut too short and has moved out of one handle into the other one. The door will only open from one side then. If it has done that then put something like a screwdriver into the square hole and turn it. That often happens when people change lever handles for hollow door knobs and use the original square shafts.