I'm so sorry that you're going through this. We just brought our cat home from the hospital for not eating. There's a long condo about it here, if you want to see what others had to say:
viewtopic.php?f=28&t=125191
BJM is right that you should check her BG level if possible, although I would hope that your vet would not send her home if her level was very high. If you have any doubts about her condition, call your local emergency vet. Ours was surprisingly helpful about answering questions over the phone about whether Tisha really needed to come in and I felt much better after asking.
Do you know if she got food while she was in the hospital? Did they syringe feed her at all? If she ate earlier in the day, she should be okay for just overnight. Cats need surprisingly little food to keep them from developing liver complications, but they do need to eat something with fat and protein in it each day. Did they give her any pain medication? UTI's can be painful, which may discourage her from eating.
In terms of food, there are lots of different things that can coax a cat to eat. My cat would lick food off of my fingers when she wouldn't eat out of the bowl. She then decided that she would only eat yogurt. I have heard the baby food can coax some cats to eat, and that some like parmesan cheese on top of their food. Basically, the vet told us that anything she eats is better than not eating at all. Just keep trying. I know that it can be heart-wrenching and terrifying to see her clearly feeling poorly, but she should get better with time, some loving attention, and appropriate medications.