Is the crop hard? Or more large and squishy like a water balloon. If it's hard, I would guess it's impacted crop, if it's soft it may be sour crop. Unfortunately, I've found both of these tough to treat. If you haven't done so already, would remove her from the flock so I could keep an eye on her, and give her plenty of water.
If it's impacted crop, and you can get her to eat, you can try feeding her live, small maggots (spikes I think they are called in bait shops). The theory is that the maggots get swallowed whole and then will eat/break down the hard, fibrous mass that her crop can't move. It worked with one hen I had (but full disclosure, she was left with a pendulous crop that never worked quite right and I still had to put her down a few weeks later). They only other thing is try to get her to eat some oils and (very gently) massage the crop.
If it's sour crop, try to get some probiotics in her, either thru yogurt with live cultures or a supplement. I also had good luck with one hen, by giving her monistat (a yeast treatment found in the women's health section in drug stores) -- there are threads on it on these forums if you search. I wouldn't have tried it, but my hen was on death's door, laying flat out, hardly able to stand and vomiting. After 3 days of monistat treatments, the crop went down quite a bit. It's been about 3 months, and the hen is still alive and doing pretty good, although her crop, too, was left large and pendulous. She's hanging tough, but not exactly thriving.