Forum:Can't finish P2 after beating DLC3

I was in the middle of P2 of main story (Mission: Bring the vault key to Tannis) when I downloaded DLC3 (Knoxx). I beat DLC3 on P1 and part of P2, but when I went back to complete P2 of the main Borderlands story, the mission I left isn't there. Even when I go to load the game and I select P2, it says the mission I am on is Bring the vault key to Tannis. But when the game loads, the mission isn't anywhere to be found. Is there any way to fix this, or will I have to join another person doing that mission on P2 of the main game?