Welcome to PT. I'm sorry to hear about your cat. First I would take her to your vet so that she can be checked out to rule out a physical problem. Then if there's nothing wrong with her physically it must be behavioral. It sounds like she may be depressed due to the loss of your other cat. Here's a great website that talks about litter box problems and provides solutions for them: http://www.catinfo.org/?link=litterbox. Good luck.