How do I fix a counterweighted accordion blind that is stuck?
This happened to one of our cordless blinds (made by Bali), they were stuck and would not raise or lower. I'm not certain that all types are made the same, but in the Bali blinds at least you can remove the top cover and there are one or more spring tensioned cylinders (for lack of a better term), and the blind cord may be wrapped around one of them incorrectly. 

It's not fun, but I had to unspool and respool all of the blind cords until all were wound correctly around their cylinders, which required a surprising amount of force. I'm sure this makes little sense as typed, so here's a youtube video that helped. Your blinds may be a little different if they're from a different manufacturer.

Bali cordless blinds won't go up or down (not my content)