During very hot weather and also when there are steep fluctuations of temperature the metal casing within the plastic doors can expand thus causing the door to affectively become jammed, depending on their set-up, it could only need to expand 1 millimetre or so to become stuck.
Although it is not a guarantee, if this occurs and you can use another door in the meantime, you may find that when the temperature cools later in the day, the internal casing in the door will cool down and contract enough to be able to open it again.
Although this will not solve the problem, it will atleast allow the door to be opened without forceful entry which in some cases could result in marks being left on the door and frame and in some cases the internal parts becoming damaged. It is definitely not a good idea to force it as you are more likely to break something internally causing
The hot weather generally effects most doors including wooden ones to some degree, but the ones most at risk tend to be the ones that are already out of correct alignment, thus when the metal casing expands it exaggerates the problem and then jams up when you attempt to operate the locking mechanism.
No matter what the weather or the temperature, should you have difficulty operating a Upvc or wooden door with a multipoint locking mechanism fitted, you should get it checked and adjusted at the earliest opportunity before the internal parts break resulting in an expensive replacement.