To fix a car's power window, first determine if the problem is mechanical or just the result of a blown fuse. If it is a fuse, replace it. If it is mechanical, identify the wiring path leading from the fuse panel to the window switch. Use a voltmeter to test for current interruption; this determines whether the switch needs replaced. Check the gaskets as well, as a tear can cause the window to stick, open slowly or not open.
To determine whether the issue is fuse-based or mechanical, turn on the car without starting the engine. Attempt to lower the windows; if none work, it is most likely a fuse. If there is no movement or sound to indicate that the motor responsible for the windows is trying to work, the fuse is likely blown. If the motor sounds or the glass attempts to move, the problem is mechanical.
If the problem is a tear in a gasket, remove the old adhesive and replace it with new weatherstrip adhesive. Small tears can also be fixed with superglue. Allow either method to dry overnight before attempting to open the window.
Another issue involving gaskets is friction between the gasket and glass. This is caused by misalignment or lack of lubrication. A motor does not have the torque required to open a window under these conditions. To fix this, use a silicone spray to lubricate the entire area.