There may be a number of reasons why you can't find your OneNote notebook. If you're working on notebooks both online and offline, it may be that they are not syncing properly, and you will need to manually sync the notebooks yourself.

This can be done in OneNote for desktop by going to File, Info, and selecting "View Sync Status".

There you can see all notebooks available to you and by clicking "Sync Now", this should restore your work.  

If you're still having issues, please visit the Microsoft Office support pages, which show solutions for more common issues with finding lost OneNote files.