If you have recently received a message from excel stating” Excel cannot open the file 123.xlsx because the file format or file extension is not valid” and you need to know what it means, and how to fix excel in order to recover your file. There are a couple of quick fixes available for this error. 
This error usually occurs when a user is attempting to open or edit an Excel file in Microsoft Office 2003, ’07, ’10, ’13, ’16, or ’19. The typical error message that pops up is “Excel cannot open the file because the extension is not valid”. The purpose of this article is to provide users with an Excel training to fix this excel error and recover your Microsoft Excel data.
Fix 1
- Open Excel and click on the “File” tab
- Click on “export” and choose ‘Change File Type’
- Change the file extension to a different file format that supports Excel and then use ‘Save As’.
- Check to see if Excel will then open the file.
Fix 2
If Microsoft Excel is unable to open the file, attempt to run Excel in safe mode. This Fix will also determine if there are any other errors in Microsoft Excel.
- Press the ‘Windows’ and ‘R’ key, in the dialogue box type excel.exe/safe and click ‘OK’
- If Microsoft Excel opens without any issues you then need to determine if any add-ins are causing a problem within Excel.
- Disable add-ins and click ‘OK’
- Start Excel normally and determine if the problem has been fixed
Fix 3
This is the ‘Open and Repair’ method that is built into Microsoft Excel as a utility that will allow users to correct errors that occur in Excel when the file extension is not valid.
- Open Microsoft Excel
- Click on the File menu and then click ‘Open’
- Under the ‘Open’ dialog box, select ‘inaccessible Excel file’ and click it
- On the Open Button, click the down arrow and locate ‘Open and Repair’ and click ‘Ok’
After Microsoft Excel has finished running the ‘Open and Repair’ tool you will then need to attempt to reopen the file and determine that it will work. Using the ‘Open and Repair’ method Microsoft Excel will attempt to locate the corruption and damages within the file and repair them without losing the information within the document.
If none of these methods work for recovering your Excel data, it may be necessary to obtain professional repair software. There are several options out there available to you from third-party sellers and from Microsoft themselves.
The software options will usually come with professional support to determine if the file itself is unsavable or if Excel itself is the issue.
A final note, you should always backup your information into two save data to ensure that if one becomes corrupted that you always have a separate save file to reopen the data.
Even if the data is not the most up to date it will at least save you the hassle of having to completely rebuild an Excel workbook from scratch.
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