I finished the settling down mission yesterday, got back from a finished raid, constructed some buildings in my settlement, and was done for the day. Tried to get into the save game menu, but it wouldn't load, after an hour I gave up and hoped that the autosave wasn't to far back. Today I started up the game, and was put in, in the middle of the raid with one more chest to go... but the raid was over. There is no raid here anymore, and so there are no vikings to help me open the chest. I cannot start a raid, but I'm missing the loot.
I think this happened, because I technically fulfilled the mission before the raid was over, and so it autosaved in the middle of the raid (This was because I had already gotten some supplies from elsewhere).
I don't know what to do, or if it will fix itself over time, but as I can't go back a mission or anything, I'm standing to lose a lot of game time.