If you are like me, who played this game YEARS ago, and continued on the same file, than chances are that you don't remember what quest is related to the main story line. My quest box was cluttered with tons of side quests.
Although the above solution is very detailed, it didnt really help me much since, well, it doesn't tell you the title of the quest:
The Path of Dawn.
For a (another) clear but detailed walkthrough, click here:http://www.uesp.net/wiki/Oblivion:The_Path_of_Dawn