I have a 16 year old indoor cat that has almost totally stopped using the litter box. Instead she has begun using the floor in the room with the litter box. Occasionally she will even go outside that room. We are at our wits end.
She may be going senile. Have her cheked for medical problems at the vet anyway. Try providing another litter box. Make sure it is in an area that is easily accessed. You can also try Feliway to calm any anxiety she may be having. vetcare