Sounds as though the cable / motor has seized up. If so you need to replace the whole assembly which consists of the motor, steel cable and window guide. I replaced them in both my rear doors and bought reconditioned units from a main dealer for about £60 each. You can pick up secondhand stuff for about £30, but there is no telling how long they will last.
There are several old threads on the forum on replacing the units, but essentially you remove the speaker cover and speaker, remove the door panel which involves a few screws / bolts here and there, put your hand inside the door and unclip the guide from the bottom of the window pane, remove the motor and guide bolts then pull the whole assembly out. It's a lot easier if you have the new assembly in front of you so that you can see what it actually looks like before you start!
The first side took me over an hour, the second side was done in 30 mins.