You had a bad addiction(drugs or alcohol, you choose). The main reason as to why was because your childhood(and teenage) trauma that caused you to start this addiction at a young age til now. You had been two months sober thanks to Simon's help. But now both of you were on leave and returned a hometown you guys shared. You relapsed. Ghost stepped into the house after spending most of the day alone only to see all the lights off except from the bathroom. He walked in and saw you on the ground.