DOOR IS VERY DIFFICULT TO LOCK OR UNLOCK
Is the door very difficult to lock?
Is the handle very hard to lift up?
Does it feel like the key will snap?
Common causes for this are likely to be that the door is well out of line and the locking keeps arent matching up making it extremely difficult to lock.
In more extreme cases it is likely to be that the internal multi point lock is on its way out.
My advice is to call a locksmith straight away, as continued operation can lead to gearbox failure.
An easy and cost effective solution to this is adjustment of the door/hinges/locking keeps.
DOOR UNLOCKS BUT WILL NOT OPEN
Is the door UPVC or composite?
Does the key turn, but then the handle feels loppy and loose, or stiff and wont move at all?
Is the door wooden with a mortice type lock?
Does the key turn but the door just will not budge?
If the door is UPVC or composite then it is almost certainly a failed multi point gearbox. As the picture shows a new gearbox next to an old one you can see where the old one has split its case and stopped working. In these cases I would have to gain entry into the jammed mechanism then replace it.
In the case of a wooden door, if the gap between the door and frame is very small then it is likely that the door has swelled, if there is a clear gap then the inside of the lock probably has a broken piece and would likely need drilling to open.
WINDOW HANDLE TURNS BUT WONT OPEN
Has the window suddenly stop working?
Has it been previously difficult to lock or unlock?
Does the handle now turn but the window wont open?
This is almost certainly a failed window mechanism. Luckily these can be quite inexpensive, so it is a case of gaining entry into the failed mechanism and replacing it.
KEY WILL LOCK AND UNLOCK DOOR BUT CANT REMOVE THE KEY
This is almost certainly a cam that has gone out of line. In the central part of a euro cylinder is what is known as the 'cam'. On occasion this will go out of line meaning that you can lock the door but not remove the key.
The easiest solution is just to replace the cylinder, although the cam can be put back in line with a bit of effort.