Take her to the vet.

As everyone else said, get her to the vet right away.  Not only is
something going on...serious or not...rapid weight loss in cats can
lead to serious complications and death (unlike us humans, who can
handle the occasional starvation diet).

That can be so many things. I hope you can get her to the vet first
thing tomorrow. Make sure the vet checks her teeth thoroughly, along
with the blood tests, etc.

The not-eating can cause liver damage pretty quickly in cats, so
sometimes that turns out to be worse than the original problem.

Warm up her wet food in the microwave for a few seconds. Also, try
Sardine Pro Plan canned cat food, it is very very fishy and can
sometimes tempt a sick cat to eat. Our emergency vet uses that for their
patients.

Let us know what the vet says,

Rhonda