Hello Samuel Janetzki,
Thank you for posting your question on Microsoft Q&A forum
I understand your notes in OneNote Android disappeared.
Here are some troubleshooting methods you can check and see if it recovers your notes:
Check for the correct the Account
- Open OneNote (Android) > Settings > Accounts.
- Make sure you’re signed in with the same Microsoft account you used on your PC. If you have multiple accounts (personal vs work), switch to the correct one.
Check OneDrive Recycle Bin (via browser on phone)
- Open OneDrive app or web > Recycle Bin.
- Look for deleted notebook folders (if they are in there) and restore them.
You can refer to this site for method in fixing syncing issue:
Feel free reply back to this post if you need further assistance, I'll be happy to help.