We'll need more information before anyone can really make an educated guess as to what might be wrong.
Size of enclosure?
Temps?
Substrate?
Does she have a humid hide?
How big is she and what are you offering for prey? Do you use vitamins? Calcium supplement?
Any recent changes in temp, defecation or other husbandry?
Now that that's all out of the way.....
Not eating is often a sign of stress which can be caused by pretty much anything
Off the top of my head weight loss could also be helped along by a parasite of some sort. Answer the above questions and we'll be in a much better position to help you out.
If all else fails... take her to the vet.