Slept with my ex-husband

My ex and I have been divorced almost a year (he left me out of the blue with no real explanation). He called the other night because his life is falling apart again. He asked to see me and drove an hour to talk with me face to face. We stayed up all night talking like old friends and then had sex. He admitted to me that he made a mistake by leaving and told me he's still in love with me. We are both pretty confused on how to handle the situation. Obvioulsy this is a rather complex situation, but what would be your first instinct?