MS stepped up their security. It used to be that brute force password crackers could unprotect a file, sheet or workbook. But, with 2013 and later, the enhanced security apparently makes file protection a tough nut to crack. So, if you are prompted for a password when you open an Excel file, you are probably out of luck.
If the protection is just cells on a sheet, or modifying the workbook (hide\remove tabs, VBA, etc), then there should be some tools out there. Google something like "excel 2016 password cracker".