Work through this in order
- Inspect the ZIP container and expected package files
- Locate xl/workbook.xml and worksheets
- Check for sharedStrings and styles components
- Open with a spreadsheet program for formulas and calculations
- Preserve the original workbook before repairing it
Use the matching tool
Inspect a local copy with XLSX File Inspector. After the check, save or repair files only using trusted software that supports the format.