What would cause a previously seemingly healthy person who had a good job and family begin to live in complete squaller? someone we know and love all of the sudden stopped paying for rent and completely left their place trashed. substance abuse?

You are right to be worried about a psychological problem, substance use (including alcohol), and the like. It would be a kindness to sensitively discuss this situation with her or him and encourage professional evaluation, although that probably is best done by a close family member or an especially close friend. Good luck to both this person and his/her family and friends.