My guess is the spring or springs are broken. They balance the otherwise very heavy door, so without them the door will seem stuck. Extension springs are on the side along the top tracks, and it will be obvious if one is broken. Torsions springs are on a pipe over the top of the door(8'wide door has one spring, 16' has two), they may take closer examination to see a broken spring.
i have 2 springs on an 8' wide garage door and even when one pops off it takes an effort to roll that door up...get a neighbor and have them help you lift the door.when you unlock it and turn the handle there are small metal plates to the left and right of the lock(chains run out from the lock handle) that pull back from the tracks, and allows you to lift it one might be disconnected...the fix see if you can remove the lock handle and assembly then you can just pull the chains to pop the lockdowns up and release the door.then theres this fix...imagine a straight line on the horizontal from the lock/handle all the way to the left and all the way too the right.measure 3"-4" back from the garage and drill a 1"knock out hole one side first see if the chain is connected then the other side
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