If you go to your Lighthouse, you should be able to select Adventure, similar to how you would select Prizes or Targets. Read through the missions and the one which "disappeared" will be on the last page (Listed individually with the regular prize missions). Note: The level of Story Line Missions you can do is based on the level of Prize missions you have completed. The higher you are on the Prize missions, the higher Story Line Missions you have. You must complete higher level normal Prizes first to unlock newer ones.