If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.

Twitter used to provide CSV exports of your data, now it's all in machine-readable JSON. How can you use it in Excel?

Don't manually re-enter data you find on the Web. Open Excel and transfer the data directly into a worksheet.

You must be able to open the corrupted workbook for the following to work: From the File menu, choose Save As. In Excel 2007, click the Office button.