OneNote is a Microsoft product, but I’ll endeavour to assist (I use OneNote regularly):
- https://support.microsoft.com/en-us/office/export-and-import-onenote-notebooks-a4b60da5-8f33-464e-b1ba-b95ce540f309
That above is how to export OneNote files in order to make a backup.
The other way you could potentially export your notes to other software, is to export them as a PDF, https://support.microsoft.com/en-us/office/export-notes-as-a-pdf-file-in-onenote-for-mac-a21723fb-6ac9-4c01-9462-e9edda15ab2d, or to try to copy and paste your notes between software.
2. Try going to the Notebooks menu (in the menu bar), and navigating to Deleted Notes, and clicking View Deleted Notes.
You could also see if your files are online, by going to Office.com, signing in with your Microsoft account, and clicking the OneNote icon on the sidebar on the left hand side. Then you should see a list of your notebooks, which you can open on the web. This would be at least a good starting point, since we at least know your notes are somewhere now, if they show up here…