It's your worst Excel nightmare: a damaged or corrupted workbook. This can happen for a variety of reasons -- and the good news is that there is a variety of ways to retrieve your damaged file.

If a file is corrupted, Excel should normally perform an automated recovery. However, if that doesn't work, there are a few other options you can try.

1. Recover or repair the file manually with Excel
The steps for manually recovering a workbook are quite simple.

1. Select "Open" from the File menu. In Excel 2007, click the Office button and select "Open".
2. Using the Look In control, locate and specify the corrupted workbook.
3. From the Open button's dropdown list, choose "Open and Repair".
4. If this is your first attempt to manually recover the workbook, click "Repair" in the dialogue box that appears.

Usually Excel will be able to recover the entire file, but it doesn't always work. If you've tried this process and you can't recover the entire file, you can still at least recover your data. To do so, click "Extract Data" in step 4.

2. Use the last saved version
If the file becomes corrupted while you're working in the workbook, DON'T save the file. Instead, revert to the last saved version of the file. Doing so without saving first should get rid of whatever was corrupting the file. Be forewarned -- you could possibly lose some data, but if you've been regularly saving your work (as you should be!) this is a lot better than losing the entire file.

To revert to the last saved version:
1. Choose Open from the File menu. In Excel 2007, click the Office button and select Open.
2. Using the Look In control, locate and specify the corrupted workbook.
3. Click "Open."

3. Try opening the file in another program
If Excel can't open the corrupted workbook, there are several other programs you can use to try to open it. Microsoft Tools is one option:

1. From the Windows XP Start menu, choose All Programs.
2. Select Microsoft Office, then Microsoft Office Tools and Microsoft Office Application Recovery.
3. In the dialogue box that appears, choose Microsoft Office Excel.
4. Click "Recover Application".

You can also try opening the workbook in Microsoft WordPad. The only caveat: WordPad will convert all the data into text, and it won't recover formulas. However, it will at least restore your important data. It will also recover your VBA procedures (macros) -- just search recovered text for "Sub" and "Function" to find them.

You may also be able to open the corrupted .xls file in Word, but again, the data is the only thing you'll be able to recover -- and even that may give you mixed results. So only try this as a last resort!