How old is it and what kind of husbandry are you using? Trembling or muscle fasiculations in geckos can be a sign of several serious metabolic diseases. Once they are trembling they are likely past the help of home care and will need veterinary help. I would try to get it to an exotics vet ASAP. If that is not an option, assist feeding and watering, herptivite, and praying may work.