How do you fix a window that does not go up in a Citroen ZX?

First make sure that the fuses are all OK. Then check the window relays. Then check to see if the other Windows move up-down. If they do then the problem is with that window only.

It could be the motor is jammed. A jammed motor can somtimes be enticed to work by holding the window switch in the up mode while slamming the door. It could be the control switch is dirty. It could be that you have to take the door panel off and check to see what is binding the lift mechanism. Many such motors have a thermal cutout when they can't close the window they tend to overheat and then disconnect - cutout. It may be that it is opened.

If the car is older there is more dirt accumulated at the control switches. You may have to take that apart or off and buy some CONTACT cleaner or tuner cleaner from Radio Shack or somewhere and spray INTO the switches while moving the switches.

It is also possible that one of the control wires fell off - highly unlikely but possible.

I would go to the library and borrow a book on how to remove the door panel.