The Excel for Mac will automatically recover files that were open in the event of a power failure or crash. When you reopen the app, they will be displayed. If you want to keep a file, save it immediately.
For more info on Auto Recovery, see Recover files in Office for Mac.
Tip: Using AutoSave and OneDrive can help prevent unsaved workbooks, and let you recover earlier versions of files. For more info, see What is AutoSave?
Excel for the web saves all files automatically, so you won't get unsaved workbooks.
For info on recovering earlier versions of a workbook, see View previous versions of Office files.
