The kitty did start to come out yesterday. Her food dish was empty, and the litter box had been used. I was able to tempt her to come out with more dry food, which she ate out of my hand. Ravenously! She was so hungry that I was able to even get her to come upstairs for a bit. She investigated, then retreated to the basement.
I think that she is afraid of the sound of my Mom moving around upstairs, and I think she is also afraid of her walker. I suggested that Mom go downstairs every couple of hours and add just a little bit of food to kitty's dish, so that she gets to associate Mom and the walker both with food.
Things are improving!