Take her to the vet ASAP. The vet will be able to tell you what the problem is and if it is fixable.
If you have calci sand in the tank, take it out and put newspaper or something else down. I know the pet stores like to sell it but it leads to impaction, which can lead to a gecko not eating. The calci sand clogs up their bodies to the point where they cannot digest any food. Also, leos get addicted to calci sand. It's like a kind of drug to them.
Sometimes when geckos do not eat it can be a heating issue. Make sure your cage is at least 80-85 degrees on the warm side.
But since the gecko is still alive, I have to say she must have been eating something in that time. Maybe some insects crawled into the cage or you left some meal worms in it and they've been crawling around? The rule for humans is 3 minutes of air/3 days of water/3 weeks of food so I would think it's similar for animals, unless her tail has a lot of fat stored up in it and she's been living on that.
As an emergency measure, you can run to the pet store. They usually will have liquid food drops for reptiles that you can buy to treat sick ones. You will have to hold her and put the dropper in her mouth. Another forum said that meat-based baby food might work, as well as mashing up crickets or worms and force-feeding.
You can also get liquid calcium supplements that you can put in her water, which she obviously has been drinking or she would not still be alive.
But you really should take her to the vet to find out why she isn't eating in the first place.
