I think I checked the message pruning to see if I hit something in there, and it's been set to 14 days but I didn't hit the delete button. I still have messages in my inbox from years ago, so I don't think that's it. I don't have any labels set up. I've also tried cycling through the different formats for viewing messages in the inbox, but just that one "thread" so to speak has vanished. I only have the message my partner sent yesterday in response. D: