Something got bound up/broken/jammed in the tracks.. so you'll need to raise the top, prop it up with 2x4s or something, and then access the tracks under the fabric in the rails.
There's a few good threads around here with pictures of how the tracks/springs/blocks work, so search for those and compare to yours.
Also, in general, try not to force the top! If it won't go up or down, there's a reason, and forcing it will generally make it worse!
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