how to fix a really bad sleeping bag zipper snag

Don't try to pull the zipper up or down its track. Pull the fabric perpendicular to the zipper.

you mean try to slide it out sideways from zip?

If you can move the clogged material sideways from the zip to get the zipper free, go for it. Most likely, you will not get anywhere with this approach. If this is the case, move the zipper in whichever direction it will move the easiest. There is no telling on this one: sometimes it's back the way it came and sometimes it is continuing on ahead. The material in front of the zipper, that is to say in the direction that you are moving the zipper, must be kept taught. The idea is to have less and less of the material stuck in the zipper as you go until it is free. Note: this can be done with 2 hands but four make it easier.

that is a nasty thing you've got there. the material is not only stuck, it is wrapped all around THRU the teeth. yikes ….

if you can grip all three pieces (left/right/ and liner) in a vise, you'd be in a good spot to be able to pull like the dickens. remember of course that some bag zippers are "lockers" and thee must have the pull tab lifted/pulled in the right direction in order to free the latch (it's a very small device. look inside the loop which retains the zipper pull).
a vise is nice because it might let you use more force, and not split any one particular layer apart.
if there are pull loops on both the inside and outside of the slider, i might try running some cord thru them, and using both at once. that would balance the force, (like deploying a T-handle wrench to more easily pull a stud).
or .. if the slider will move even a tiny bit, you might consider making a small slit in the liner, and steering the slider into it towards freedom.

good luck !
v.

pull fabric sideways while you're moving zipper pull back and/or forth

"pull fabric sideways while you're moving zipper pull back and/or forth"

That has always been my approach. It takes lots of patients as a snag like yours could take 30 minutes to undo. Keep at it, and accept very small improvements. Don't get frustrated and try to just rip it out of there.

Good luck.

I had that happen once when I was going to bed. I fixed it with a knife and duct tape. It's been just fine ever since. It's a good thing I got it unstuck. Lows were below zero that night.

couldnt figure out how to do it so i took it to rei and the guy there did exactly as was said here-he literally pulled it out sideways/parallel to zipper and it came out…took him about 5mins of super hardcore pulling(i would never conceive of using such force on such thin shell material but thats how its done!)…amazinly no tears in shell.Just in time for my snowy trip this week.