They split into three categories. When you reopen the file, Excel will first try to repair the file itself. The second step is to try and extract all the formulae, leaving you with raw data. Then there are some manual options to try.
As with anything else, you should always start by looking at the simple options. The most basic of these is to simply reopen the file. If that doesn’t work, restarting the computer can sometimes make a difference, as can opening the file in Open Office. You will then be able to save it under another name before trying Excel again. You could also open Excel in safe mode and scan the file for viruses before opening it to make sure it’s safe to use.
After you’ve exhausted these simple approaches, there are still other things to try. In case anything goes wrong, it can be a good idea to first transfer the file onto a backup hard disk so you’ll have a spare copy. Then, as long as you can open the file in Excel, you can use the Revert to Saved Document command. This should bring up an older version of the file, which can be saved in SYLK format and then HTML format, which should help repair the data.
If, however, Excel won’t open your file then your next step is to try some alternative manual options. Opening the file in either MS Word or WordPad can help. Do this by changing the .xls in the file name to .doc so it will open in Word. This should then let you recover your data by opening the file in Excel viewer.