Good morning to you...it would be best for you to take your kitty to the veterinarian as soon as possible. Cats don't tolerate going more than 24 hours without food very well.
Is she not using her back legs at all? Has she gone outside in the last 24 hours? What about the litterbox? Has she urinated or defecated recently?
If she is not wanting to use her back legs, she needs to be seen.
Let us know what you find out...
Tomcat
I hope you have already taken her to the vet, since this could be serious. Loss of leg functioning could be caused by a stroke or blood clots...that happened to one of my cats when she was only 2 years old. The emergency vet thought she had broken a leg, but couldn't find any fractures (this was in the middle of the night over the weekend), the next morning, when she lost functioning of two more legs, she had to be put to sleep. It still makes me cry just thinking about it...
I really hope this is not the case with your cat, but I definitely wouldn't take any chances. If you haven't already, please take her to the vet ASAP!
It could just be arthritis setting in. Let us know what the vet says. If it's not serious, and just joint inflammation, then there are supplements that can help. Otherwise...let us know what your vet says.