There are three possibilities how a story can "disappear" and how you can find it again:
1. The story expired.
Each story has an expiration date. By default it is set two years after creation. But it can also be modified individually while creating a story. At the expiration date, the story automatically moves in the "archive" (as far as an archive exists). The menu specified as an archive can be viewed and edited in the Menu Settings
under Website & CMS > Menus/Settings
2. The story was moved to another menu by accident or it is not associated to any menu anymore.
Under Stories > Edit
you have options to relocate the story. Choose from the selection box Selection
the single options like "Select stories posted by editor" if you know the editor of the story or "Select stories with no assignment to a menu" if you don't find the story in the regular menus anymore.
You can also locate the story by using Global Search & Replace
3. The story was overwritten while working in two browser windows.
Basically you should never work in two browser windows because only one window is active and that's how content in the other window can be overwritten. But with the Version Management inside the stories you can revert to the original version.