There are a lot of things that could cause the symptoms you have described. For example, there are quite a few different viral infections that cause a lack of appetite. Another relatively common disease is stomatitis (inflammation of the mouth lining) plus a problem with the intestines that can both cause a lack of appetite.?As you can see from just these few causes, it is important to get your gecko checked out by a vet, preferably one that specializes in reptile care.