My dog is 12 years old, a shepherd mix, medium size female. She stopped eating a few days ago and nothing I try to feed her tempts her, even her favorite foods. She also seems quite listless lately. Other than that, I haven't noticed any problems.
Please help!

Try to get her to the vet yet this week. It could be something easily cured. For more details, see the sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171

Tempting her with rich food will not cure what is causing her not to eat.

My pug mix started behaving similarly last year at age 9. Her energy levels went down considerably and she slept a lot. We thought she was just getting older. Then she started peeing in the house out of the blue. I took her to the vet and discovered she had diabetes. Now she gets insulin 2x a day and is back to behaving like a puppy again.

Good luck.