Great that you noticed it right away, and I'm sure you're going to call the vet first thing tomorrow morning anyway, even if we weren't all urging you.
These things always happen on weekends, when the vet's office is closed...
In the meantime, try smelly food. "People" tuna, sardines. Try warming her food a little, to make it smellier, by placing it in a hot-water bath — don't microwave it. When cats can't smell, they won't eat, even favorite treats you think they'd recognize on sight. Try Stilton!
Have you looked in her mouth? And do check the litter box; your vet will want to know if anything is off.
You don't want this not-eating to continue for any more than a day, especially at her great old age. Good luck, and please do keep us posted.


