After playing the mission for the first time, I ame off it, and when I went back to go on it again, it was missing, and has been since.
Any help?

Ah, sometimes glitches like this happen. It can be for Various reasons. do you have any specific details? like, were you running anything else at the time? if so, what was it, and what were you doing with it? was the CC folder open? Was a certain something-or-other glitching? ect.

Edit: By the By, I love your username. Crumpets... great choice, if I may say so.