Sequence and Footage has disappeared!

Hi everyone!

I had a project that I was editing, on and off, over a few weeks in between other work. Never had any issues with the program over that time. I just got back from being away for three weeks, the project file is there but the specific sequence and footage I was working on has all disappeard like it never existed. No trace of it whatsoever. 

Any ideas why this might be? 

I've accpeted that I'll have to start again, I'm trying to understand how this could even happen!