Hello and thanks for trusting me to help you and Heather today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
There are various possible reasons for Heather's actions today. Older cats are in a very fine balance and it doesn't take much for that balance to be upset. Sometimes with a little supportive care, we are able to get them back to where they were, but other times, it may be that their organ function has deteriorated to the point that they may no longer function. It is often hard to distinguish without trying and evaluating their response.
Another possibility is that her lack of movement may be due to the pain of degenerative joint disease (DJD) or arthritis. While it would seem that would be something that would not present as acutely, it often does as the chronic pain builds up and finally gets bad enough that they just don't want to move.
The best course of action would be to have her evaluated by your vet to see if there is anything that can be offered after a thorough physical exam. If she is still eating and drinking some, this need not be considered an emergency unless she seems in pain or distress. In the meantime, I would do exactly as you are and allow her to rest and do what she feels up to until you can have her evaluated.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.