So about 11 hours of play time ago I entered a town at night and it was under attack by vampires. During the fight I accidentally shot and killed a Dawnguard member. Shortly after I received the quest to meet the Dawnguard leader. Today I finally got around to beginning the quest chain. As I enter the area for it everyone begins to attack me. I guess they were still mad from me killing that one guy. But now I can't do the quest line because they all want to kill me. Does anyone know how to fix this problem?

The only thing I can think of is revert to an old save

I thought of that but like I said last one I have is over 11 hours of game play back

Ok, found out the problem. I had a bounty in Riften of 40 gold. I paid it and now they are not attacking me and I can continue with the quests. So it seems any bounty in Riften makes them attack you.

Yes, they also enforce law in Riften for whatever reason.