Hi. I think it's quite likely she has eaten something she shouldn't. This sounds like a reaction to mild poisoning - probably from the trash. Do you know if she ate anything like chocolate, onions, or anything from my list - see article at: http://www.infobarrel.com/Everyday_Foods_That_Are_Poisonous_To_Dogs
There are of course other possibilities, but my best guess is a digestive upset. I assume she is eating okay and not off her normal food? The fact she won't drink is slightly disconcerting, as it is important to keep her hydrated. There is an old trick, which is so simple, but should only be done with great care ... give her ONE (no more) potato crisp ... dogs will usually eat these easily and happily, and then want to drink because of the slight salt deposit that stays in their mouths. Only give one crisp though, as more is not advisable.
The fact that your dog is lethargic is indicative of something troubling her - have you seen her stretching more than usual? This also indicates a stomach upset.
If this continues for more than the next 24hrs, I would definitely advise seeing a vet. If it was something she ate, it would have passed through her by then - but if it was a non-food item she has eaten, it could have caused a blockage (bloat) or internal injury, which would need investigating and possibly also X-rays.
Let us know how you get on. Tony
I would not wait on this because if she does have something serious like a foreign body obstruction, time is of the essence. If it is just a GI upset, your vet can give you something to ease her pain. Good luck.