It's probably nothing to mess around with. Turkeys sound like they are a lot more fragile than other birds.
First, is anything different -- diet perhaps? Is the weather extremely hot -- could it be overheated or dehydrated? If so, change these factors and see what happens.
I'm certainly no vet, and don't have much experience with turkeys, just having gotten my first ones this year, but based on my experiences over the years with chickens, ducks, and various other birds I would do the following:
1) Isolate the bird someplace very warm. Preferably around 90 degrees. If they're sick, heat helps their bodies to fight it off by putting less stress on them. Don't overheat the bird, of course.
2) Give it an electrolyte solution instead of plain water. Gatorade or Pedialyte is fine. Or, you can mix about a tablespoon of sugar or honey in a quart of water, then add a pinch of salt. You could also add about a tablespoon of apple cider vinegar to this mixture.
3) Give it antibiotics right away -- you can mix them in the electrolyte solution if it will drink. Tetracycline and either penicillin or a cephalosporin type would be a good combination to use if you don't know what the specific problem is.
4) If it will eat, feed it something high in protein and easy to digest -- game bird crumble and boiled mashed eggs would be good. Some yogurt might help, too, to add probiotics. If it won't eat, you could try force feeding if you can do it without unduly stressing the bird.
I hope this helps.
