If you have a password protected workbook, you can fix this when you forget your password.
Backup your excel file
Change the extension from .xlsx to .zip
Open the zip file with 7-zip
Goto the directory xl
Open workbook.xml in edit mode with notepad
Find tag <WorkbookProtection>
Highlight tag <WorkbookProtection> to </WorkbookProtection> and delete.
Change the .zip to .xlsx
Open excel file and it is now unprotected.