All Media disappearing from sequence

This is an issue I've had a few times. My editor and I are working on a team project, and all of a sudden 3-5 of our sequences lose all their media. The sequence is still there, but the preview is black and all the media is gone. sometimes it still has its captions, sometimes it doesnt. 

It dosn't seem to be an updating and pulishing error, because neither of us had messed with those sequences recently. 

this is infuriating, and it loses me hours of work whenever it happens. 

Does anyone know how to fix this? I can go back into the auto save or version history, but that still doesn't have all the changes we made.