Frequently it is the rubber on the windshield frame is stuck to the windshield opening. Take a piece of plastic, like a plastic spatula, and work it between the rubber and the body. When you can get a piece of string to go through the opening, run the string around the windshield rubber/opening, and free it up from the body. Then try your windshield regulator. Easy does it as you do not want to strip gears. If it opens, great. If not you will need to remove the regulator and try to free it up with ATF fluid and Acetone or some similar penitrating lubricant. Good luck.