Hi Allie! Maybe I can help.
Cats do feel stress, and can be stressed by many things, even the smallest details you wouldn't expect to bug them (moving furniture around, for instance!). Since it has only been a day, I wouldn't freak out about this too much, but if she continues this behavior without coming out to eat anything, even tuna or her favorite morsels, then a call to the vet would be in your best interest.
Since she is 8, she is considered a kitty senior citizen. Cats can start to experience health issues at this age such as kidney disease, diabetes, thyroid issues...have you noticed any behavioral or health changes aside from the sudden hiding?
She's always been a very quiet cat, likes to sleep and spend most of the time curled up on my pillow. She did look unusually tired this morning.
It could just be a bad day. Just like you and I, they can have a day where they just don't feel that great, or want to be that active. Having 11 cats myself, I can tell you, they are all over the board from day to day!
I wouldn't get worried unless she doesn't eat or drink for 24 hours. What I recommend is moving her food, water, and maybe some snacks she can't ordinarily resist like wet food or tuna into the room where she is hiding along with a littler box, unless that is in a room she can easily get to from where she is (some cats HATE when you move the box!).
The best thing to do is to leave her alone as much as possible for a few hours, and only check in on her eating progress. If you talk in the room, speak soothingly an calmly. She may be looking to de-stress, which is why she chose a small, dark spot in which to hide.
If by tomorrow morning she hasn't touched food or drink, ring the vet and inquire about senior kitty bloodwork. It's possible something is going on that she's not showing obvious symptoms of as of yet. After a cat turns 8, it's not a bad idea to have their bloodwork done every 6-12 months, as metabolic changes can come on rapidly in animals.
Okay. I appreciate your advice, and I will follow it. Thank you, Jes. :)
Let me know what she's up to in the morning, Allie. Keep checking in on her periodically to make sure she looks OK. If she doesn't come out and lets you hold her, check her gums to make sure they look nice and pink. If they look pale or white, call your emergency vet tonight. I doubt that'll be the case, but I wanted to throw that out there just in case you notice anything else after we are done chatting. Hit me up anytime!
If she DOES come out, that should have said!