Assuming you can get to the inside: Take handle off (Use a DECENT screwdriver that FITS PROPERLY).
You will see a square shaft that transmits the drive from the handle to the workings of the latch.
You may get some clue what has gone wrong by looking at this: Is the square hole in the back of the handle still square?
Is the shaft long enough to reach inside the handle shaft or has it been over-enthusiastically trimmed?
Is the shaft still square, especially where it needs to drive or be driven?
Is the hole in the lock mechanism itself still square or has it become rounded and the shaft is just spinning?
You may well be able to do some tinkering with the shaft and handle or a pair of pliers even, enough to get the lock moving.
Ultiumately you will possibly need a new handle ad / or a new chaft - maybe a new lock.
There are plenty of places selling spares for uPVC doors - Google is your friend. Just make sure you measure once, measure twice, go away, measure twice again and buy a spare only once.
XXX, July 2016
