Your catis a pretty fastidious animal. Thus if she stops using the litter box, you know something is amiss. The most common explanation is that her litter box is too dirty and smelly. If the litter box disgusting, your cat will find someplace less appropriate to go. Your cat may be stressed and avoid her litter box. Household guests or new household members, including other pets, can be stressful. Your cat can be disturbed by construction in or around your home. Stress may be caused by something even more simple such as moving the furniture around. Consider setting up a second litter box where your cat hangs out while she works through her stress. Of particular concern, your cat may stop using the litter box because she doesn’t feel well. If you can’t find an obvious reason for litter box avoidance, make an appointment with your Pleasanton, CA vet for evaluation and possible treatment.