Hi Luc -- I'm sorry you have a student reporting a missing story. Does she have any information about the story like the link or title? If she published the story to everyone before it disappeared you can try to locate it by search.
We haven't been able to reproduce this issue where a story is created and then no longer appears in someone's account, but we've noticed a pattern that this issue is almost exclusively reported by students. We know that students often use public computers or a friend's laptop, so we have reason to believe these cases are caused by the student inadvertently creating a story in someone else's account when using a computer others are using to do work on ArcGIS Online. When they log into their own account later they don't see their story since it was created in a different account.
Our current advice is to reiterate to students that they should ensure they are logged in with their own account before starting to do any work on ArcGIS Online, and also be sure to always log out when they complete work on a shared computer.
If you or the student has any other information about the specific steps or conditions that led to this situation, please share them with us and we can investigate further.
Owen Evans
Lead Product Engineer | StoryMaps