Forum:Where'd he go?

well ive been playing borderlands for the past week,and ive finished basically every mission but last night i got to the final boss but decided to leave it till tomorrow,as it was pretty late so i turned my 360 off and when i came on to do it today the boss didnt spawn,now my map marker points to the two columns where it appears but it just wont,it says the objectives complete,im really annoyed and dont know what to do help!?