Enabling History
By default most notebooks do not have history enabled, this means that notes can easily be lost, especially when moving between classes.
To enable note history
Step 1 - Open the notebook in OneNote for Windows 10
Step 2 - Click on View
Step 3 - Click on the small drop down arrow next to deleted notes
Step 4 - Make sure Disable history for this workbook is unchecked
You can also click on Deleted Notes to view any notes that have been deleted.
This process can also be done online or with the older version of OneNote but the steps may vary slightly.