Check if the workbook or sheet is protected
Open the Excel file
Go to the sheet tab at the bottom
Right-click the sheet tab
Select View Code (if it appears) or Unprotect Sheet (if available)
If prompted, enter the password for the sheet
To unprotect the workbook: go to Review → Unprotect Workbook
If you have no password: contact the file owner/administrator
If the file is protected but you have the password: enter it in the Unprotect prompt
If the sheet is protected for editing only: go to Review → Allow Users to Edit Ranges (if you have access)
If cells are locked individually: select the locked cells and check Review → Protect Sheet options (if you have permission)
If you need to unlock cell editing: go to Review → Unprotect Sheet, then format cells to unlock (Home → Format Cells → Protection → uncheck Locked), then re-protect if required