Hello Koen, thanks for contacting us and letting us know about this issue. I'm sorry it happened to you.
May I ask you, were your notes synced to a sharing app (Box, Dropbox or SkyDrive)? In case it is Box or Dropbox, you can recover the previous version of your file.
If your notebook is stored locally, we could try to get your data back. To do so we need the problem section file and page name where data are missing. Please send it to email@example.com. Sure if no private info is involved.