take it to the vet ASAP. When you take an animal in, it is your responsibility to care for it, especially in it's time of need. Don't worry about the cost until you get there and find out what is wrong with it. If you can't pay for it, leave the snake there and the vet will deal with it (which unfortunately may include euthanasia). Don't just not treat the animal because you can't afford to take it to a vet - if you do that, you have NO RIGHT to own any animal. Vet trips and emergencies are part of being an animal owner.
Good luck with the snake! Hopefully it's nothing too serious.