upvc door lock turns but does not unlock?

I'm guessing you've tried key both sides so it's likely to be the lockcase rather than the cylinder.
Part of the diecast mechanism inside will have snapped.
The door may have to be drilled underneath the handle to be opened but will then need the same type of lock to secure, there are hundreds of makes and sizes.
Next question is why did it break? chances are the door will need repacking and the hinges adjusting or your new lock will soon fail too.

Amlocks
February 2012

Drill through barrel & renew not easey

Len
January 2012