important notes disappeared for no reason

Hello everyone, 

My girlfriend has some important exams coming, and a few days ago, she logged in to see that all of these had simply disappeared from her onenote. I already checked some eventual other accounts they couldve been on, and the deleted files history ; I simply cannot seem to be able to find them. These were not made in a single session, meaning it couldntve simply not been saved on the cloud. Has anyone got any idea of how I could try to recover these notes ? 

Thanks in advance