If your lock is a FHUR, I'm guessing it's a PVC door, and it will be a euro-cylinder lock.
First, check that the door won't lock at all even when you lift the handle right up when trying to turn the key. If it does, then your door needs some minor adjustments.
But if as you've said, the key won't turn at all, it could be either the cylinder that has worn/become faulty/just got a bit of crud stuck in the key slot on the inside. Or even just the key that has become worn.
Firstly, try another key if you have one.
If no joy, you could try cleaning the cylinder, which is easiest done when removed from the door, which is very easy to do, provided you can unlock the door & turn the key from at least 1 side.
There is a screw on the edge of the door which lines up with the cylinder, which you remove (the cylinder will become loose), then turn the key through about 30 degrees, and the cylinder will then just pull out. Give it a shake, a blow or what ever, and put it back & see if that has worked.
Failing that, you could try replacing the cylinder itself, which is as easy as described above. As good a place as any to get one is
Screwfix, but make sure you get the correct size for your door. The easiest way is to measure the length of it while it's removed. If you get a longer one that sticks out a bit, it can be a security risk.
Hope that helps.