Look at it (use binoculars) and determine how and why it is jammed, then work to resolve that specific situation. The one time I jammed mine was because leech tension was insufficient (not enough vang/mainsheet - I was playing around to see how much inappropriate handling it could take - great learning!
), and my 16 year old main (in dire need of replacing) has a very floppy leech, so the leech folded over and got furled in that way.
To unjam that, I furled it out as far as it would go, then jiggled the furler back and forth (you can put a winch handle into the winch at the mast), which got it out somewhat more, but then it was jammed tight at the top spreader. I then let the halyard go and took the sail down, sliding it out of the luff groove until the jam arrived at boom level (the slot is the same width the whole length, so usually a jam even partially furled can be slid down and pulled out), where it was fairly easy to sort out. If you can get it neither unjammed nor take it down, someone will have to go up and tug on it there.
Also if you have an adjustable backstay, let all the tension out of it so the mast is straight, and what also helped was putting the outhaul tight and then furling
in some more, which tightened the coiled up sail from the inside and thus made the whole sausage a bit thinner.