Please help our baby is very sick...updated

One of our cats, Daisy, seems to be very sick. She is 3 years old with no prior health problems. We called every vet in town on Friday and we were unable to get her in. We have no 24 hour animal hospital either.

I don't have any idea what might be wrong with her. She is laying on the floor in our closet and hasn't moved in two days. She won't even stand up. If you try to make her stand, she lays right back down. She is not her normal, busy self at all. Normally she goes right to where the action is.

She is getting worse. Friday, she was hiding in the closet. She would still get up, go eat, etc. but she would go right back. This morning, she was more lethargic, now she won't even move.

We have isolated her to our bedroom with her own litterbox. She hasn't been eating and she drinks very little. She urinates but has not defecated at all. She doesn't pur or meow which is very strange for her. She is not heaving, or making any noises at all, except for what I would call a "sigh" from time to time.

We have not changed her food, she is an indoor cat that has never been outside. We only have one other cat, also an indoor only cat, who is fine.

She has only vomited once and there is no sign of diarrhea.