The majority of call outs I get is down to failed centre cases, also know as gear boxes.  You will find these in the side of your UPVC door, the spindle and cylinder are both attached to this.  It's very common for these to fail  either when the door is locked so it can't be unlocked or when its unlocked and then can't be locked.  There are lots of different gear boxes out these depending on what type of mechanism you have in your door.  I stock most of the common ones so I can replace them on the spot.  The others I don't have I can get hold of if they haven't been discontinued.  If they are no longer made there is normally an alternative but this may mean replacing the full mechanism or locking strip which runs the full length of the side of the door.  Please contact me if you think this has happened to you and I can normally come round on the same day to give a free no obligation quote.