My back window will not roll down. What could be the problem?

First off I am going to assume since this is 2015 that you do not have hand crank windows. SO: like all garage door they are way too heavy to be lifted by a person or by a motor opener (GDO). So the garage door has powerful coil springs that do the lifting and lowering, the GDO simply provides direction. Assuming that you have checked fuses,get inside the vehicle and when it is completely silent click the up/down button. Do you hear the motor attempting to move? If so then it is a broken coil spring. If you do not hear the motor, then put a volt meter to the wires, and see if you get 12v when you click the up/down button. If you do, then you need to replace the motor.

BTW: if any of your windows are moving up/down slowly go to the store and buy a can of silicone spray (NOT WD-40). Spray that on the black rubber seal all around the window edges and let it dry. Your windows will now move up/down easier.