Hi Libby,
Sounds like a respiratory infection.
Amongst other symptoms, the main problem is the thickening of saliva. Because snakes can't cough, this leads to a build up, and furthers to difficulty breathing. If left untreated, canker generally forms, eventuating in death.
If the snake is still flicking it's tongue, and the tongue forks aren't stuck together, up the temps into the high 30's at the hot end, ensuring ample water down the cool end. If you see no improvement within a couple of days, take it to a vet.
If the snake isn't flicking it's tongue, or the tongue forks are stuck together, I'd suggest you take it to a vet at your earliest opportunity.
Cheers.