Hi David,
Thank you for posting your query in our community.
Based on your description, you want to convert back from Excel back to Table in OneNote.
If you don't make any other changes to the notebook after you accidentally click the button to convert it into an Excel worksheet, you can click the Undo icon(Curved arrow) in the Quick Access Toolbar to withdraw this action.
If you can't find "Convert to Spreadsheet" option under Undo icon, then I'm afraid that you can't convert it back. You need to re-insert a table and copy-paste the content.
Appreciate your understanding.
Best regards,
Jazlyn