We are going to have to have you look at the door and the hardware, and provide some observations. Usually when this happens, a counter-balance cable is either broken, or has fallen off the drum-spool, or the springs may have broken.
Take a look at the counter balance rig and note whether the cable on the side of the door has unraveled from the spool on the bar above the door or broken, and if the springs are in one piece over the door. There are several possible configurations of springs and counterbalances and it would help to get a good description of yours.
Finally, before we get into repair methods, because these parts are not easy to replace or repair, let us know if you feel up to dealing with this fairly challenging task. If not you may need to call a garage door company.
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