Cat Not Using Litter Box?

I have six cats, but one of them lately has not been using the litter box and has been peeing and pooping all over the house even though we have enough litter boxes and going in them has never been a problem for her. My family thinks it began a few months ago when she had a near-death experience with a neighborhood dog and spent a few weeks hiding under beds, where she refused to come out and now has more or less forgotten to use the litter box. I don't know what to do, but it's making the house smell and I'm getting tired of cleaning up after her. She has no health problems that we can see, and she seems to have completely recovered from the traumatic experience, and we have not changed the type of litter in months or the boxes used (a few are covered, and one is not). Anyone have any suggestions before we permanently banish her outside?