If both halyards jammed, it could be something about the sheeves or the pin they turn on has jammed. Or, one line jumped a sheeve and jammed them both. Ascenders are great but for a one-shot, you can use climbing loops made up with Prussic knots. You'll find information on the web about them, or at a local climibing shop if you have one.
All you need are three loops of 1/2"-5/8" line, tied with square knots or other secure knots. One about 6' tall, the other two about 8' tall. You throw all three loops over your climbing line, and use the two long ones as "stirrups" and the shorter one as a chest (under the arms) safety. Standing in one leg strap, sliding the other one up a foot, transfering up into that, bringing the other leg strap back up, keep alternating and you're walking up a halyard, pushing the prussic knots up as you go. (The third loop around your chest is there as a safety, in case your foot slips. Don't omit it.) Prussic knots tighten and grab under pressure, but pop loose with a thumb so you can slide them up. They're what climbers used before ascenders were affordable, or available. They can be used on any tapered spar--including many masts--as well.
As usual make sure there's a safety line besides your climbing line, you never want to fall when that "stuck" line decides to get unstuck!