Corrupted workbook recovery

Before trying anything else, make a copy of the workbook and keep the original unchanged. If it is on a network share, USB drive, or cloud-synced folder, copy it to a local folder first and work on the copy.

In Excel, go to File > Open > Browse, select the workbook, then use the arrow beside Open and choose Open and Repair. Try Repair first. If that cannot open the file, repeat the process and choose Extract Data to recover values and formulas where possible. Save anything recovered under a new file name rather than overwriting the damaged workbook. 

If the file is stored in OneDrive or SharePoint, check Version history before spending too long on repairs. Right-click the file, choose Version history, and restore or download a version from before the problem started. That is often the cleanest recovery option. 

If neither option works, avoid repeatedly opening and saving the original file. Check for a backup copy, an AutoRecover file, or a previous version in File History. The Excel version, file type (.xlsx, .xlsm, etc.), storage location, and exact error message would help narrow down what can still be recovered.
Here are two Microsoft guides that may help:
https://support.microsoft.com/en-us/excel/repair-a-corrupted-workbook
https://support.microsoft.com/en-us/excel/restore-a-previous-version-of-an-excel-file