very serious, he could have injested something that is now stuck, or have any number of other things, if she is older it could be a sign of cancer. if she is not up to date on shots it could be parvo. is there any chance she ate something toxic
I agree, dehydration can kill. take her please and let us know.
Once a dog refuses water you have a life-threatening situation. Please waste no time in getting her to an ER vet!