I managed to nurse a young (4 inch?) leo back to health from emaciation after his caretakers at a petstore put a UVB bulb over him with NO heat source whatesoever, cage temp was lucky to reach 70F
He hadn't eaten in at least a month by all accounts and had trouble walking. I cleaned out the enclosure, removed the substrate replacing with newspaper, added an undertank heat pad and a 60w bulb and tinkered with it until he had a nice 85-90F hide area and good ambient temp of 76F. I also put a dish with powderded calcium supplement near his water (with electrolytes) and syringed him some warm electrolytes. As soon as he warmed up he perked up and tried (with much effort) to hunt very small gut-loaded crickets liberally coated with vitamin powder. I fed him 1 or 2 said crickets as often as he'd hunt them (about 4 times a day) for several days and continued trying to syringe electrolytes but if he fought me I put him back for fear of stressing him too badly (he was also not used to being handled, ever).
I also tried various worms and syringing him worm/electrolyte mash but he did NOT like that as all he had ever been fed was crickets. Not sure how much he ever actually ingested.
Within a week he did start eating live waxworms but remained reluctant so mostly what he ate was dusted crickets. After the 2nd week he was HUNGRY at all time so I still fed him 3 or 4 times a day, still small meals, and he started getting his color back.
Within a month he was being fed only 2 or 3 times a day, was putting on weight at a pretty steady rate but still a little thin, colors looked good and vibrant, still reluctant to eat other bugs but would at least once or twice a week if that's all I gave him at a feeding.
After 3 months he was looking good, active, eating well, used to handling by now of course, started growing again(!) and got a new home with a former gecko keeper that had been rooting for him.
Last I knew, about 6 months after I took him in, he was growing like a weed with his new owner, seemingly to make up for lost time, and spoiled rotten. They pretty much kept him in the same setup I had to avoid substrate ingestion and allow constant access to electrolytes and calcium.
The keys to bringing an animal back from starvation are hydration first, then gradually easing them back into a normal diet.
Use oral electrolytes if you can and definitely add them to the waterbowl. NEVER syringe an emaciated or shocked animal cold fluids! At least equal to their body temp or slightly warmer.
Introduce food and regular feedings slowly or it can cause "re-feeding syndrome" which can shock their system and be fatal. If the body is not used to digesting food or being nourished the sudden introduction is stressful on the body.
Very small meals several times a day until they start regaining strength, then slowly up the amount on the same schedule or same amounts more often until they can handle larger meals. Forced or syringe feeding may be neccesary, mash whatever you are trying to feed them (helps familiarize them to the taste as well) and add water with electrolytes until it is a workable consistency, load your syringe/spoon/whatever, and try gently to get it into their mouth. I generally have luck with a syringe feeding but that's just me. Lots of experience in it with birds.
Hydrate and refeed slowly in as stress free an environement as possible.
I've had luck using this general method in cats, dogs, sheep, cattle, birds/parrots, rabbits, rodents, geckos, a snake, and a tegu.