You may have a stripped out gear but be careful because you may have a broken spring that caused the gear to strip out in the first place. First with the door in the closed position i would pull the disconnect cord from the opener. Then make sure that the door goes us and down manually without too much effort. If you can't open the door at all then the spring is probably broken. If you can lift the door but is seems to be binding then that's were you should start to address the problem. Feel free to shoot me an email or visit my site and contact me there at
www.gdrepair.com