Oh dear that sounds either like sour crop or a blocked crop. I have had to put down hens with both.
Not sure about sour crop but when the crop has got blocked you can try syringing a bit of olive oil into their mouths, massage the crop gently for about 10 mins then hold hen upside down to induce vomiting to try to clear/block the crop. It didn't work with my hen. She wasn't actually showing signs of illness with this and it can be cleared by cutting open the crop (best not done by novices!) to clear out the blockage.
However the one we put down for a friend was showing all signs of illness, not eating, standing around quiety and sadly with eyes closed and when I picked her up yellow goo came gushing out all over me.
Susie Gee has (I think) has had experience of sour crop.
In the meantime look on one of the poultry forums because there is tons of info there - that's where I go when I have hen maladies to research. I'm just going to see which was the one I got the info about crop problems from - be right back.