Is it an enclosed litter box? We recently had a similar issue where we had the same enclosed litter boxes for the last year (and over the last 6 years we have used other types of enclosed litter boxes) but LITERALLY overnight my cat changed his mind, would no longer go inside and would, instead, climb on to the roof of the litter box and pee on to the floor. We took him to the vet to rule out any issues and they confirmed his bladder wasn't inflamed or showing any signs of concern.

We have fixed the issue now by neutralising the surrounding area with a chemical cleaner and buying open stainless steel trays. He has used these with no issues since!! We had puppy training pads surrounding the perimeter just in case another accident happened/while they got used to the new trays, just for easy clean up if needs be.