Have you checked the whole tank for poop? If it's a planted tank it may have fallen in the dirt, or be stuck on the wall behind a plant or something.

Some others have mentioned plain Pedialyte (dissolved in much water) to "perk up" a skinny or dehydrated frog. Maybe do a search for that?

Are you comfortable with force feeding? If not call a vet and see what they can do or show you to do. Don't wait til it's too late. Good luck.