there are lots of posts on here about sour crop which seems to be a secondary problem when they get an impacted crop. Basically stuff like grass or hay/straw get stuck in there and after time it ferments causing the crop to be "sour". You can treat by giving mineral oil to break up anything still in the crop (or vegetable oil), massage the oil in the crop. If you have impacted crop that is all you really need to do (it only took 2 days of oil and massage to help my hen) but once it becomes sour there is more to it. I think that requires not feeding them so the crop empties and then yogurt to get the natural bacteria and acidity back where it belongs. Apple cidar vinegar in the water will help get the acidity back in the crop.
do some searching on here about that as I could be wrong or missing parts..I have only dealt with impaction, not sour crop (yet). Sand can also cause a bit of a blockage too if they eat a lot of it. If they don't have grit out for them to eat then it is easier for the impaction to happen. Good luck!
edit:
If she has been a regular layer and suddenly is not laying, I would be concerned about a problem with her laying like egg bound, feel her pelvis and see if you can feel and egg stuck in there. Chickens exhibit a lot of problems in the same way so you will want to thoroughly check her out and see if you can see where the problem is