Cats that are trained to use a box don't often stop without a reason. Have you made sure the box is large enough for the cat? Are you keeping it cleaned out consistently? Have you changed the type of litter? Some cats are picky, picky, picky. Have you acquired a new pet? Is your cat sick? Cats signal urinary tract infections and problems be peeing in inappropriate areas. They can't speak but they can sure "mark territory" and demonstrate their problems in dramatic ways. You will need to clean the area the cat has chosen as the new area with urine cleaners you can buy at places like Petsmart or Petco.
My cat did that whenever we were gone for"top long"(a day or more). She KNEW what she was doing! It was her way of 'getting even'- Like"you better not leave me again". If thats the reason-try playing the radio ehile you are away----maybe its too quiet when you go. Just a thought.
If she does it again you are so posed to rub her nose in it ( I know it sounds so mean but it will tell them " ok if I do it again ill be punished",kinda like a child) then for a little while put the litter box in that spot. Slowly move it to the bathroom( or wherever you put it) so the next day move it a foot away and the next day and so on. My cat used to do this but we used this method and it worked. But if she still keeps doing it day after day, I would bring her to the vet cause it could be a infection.