OneNote - How can I recover from an accidental Section deletion?
This was so easy, I'm surprised no one answered!
In OneNote 2010 (I assume is similar for other versions), open the backstage (File menu) and look to the right. There's a button labeled "Open Backups"!
On my system, this opened Explorer to:
C:\Users\kevinbuchan\AppData\Local\Microsoft\OneNote\14.0\Backup
Recovery was easy now!