If you were in the middle of working on an Excel sheet and have not saved your work recently, and then Excel closes without you saving your work, you may be able to retrieve it.

AutoRecover

A feature called Excel AutoSave should be enabled by default on your computer. This allows you to use AutoRecover.

To recover a file, open Excel, click on File, then Open, and choose Recent Workbooks. Scroll all of the way down under the Recent Workbooks List until you see Recover Unsaved Workbooks, then click on it.

A window will open which contains recent autosaved versions of files you have worked on. If your file is listed here, select and open it, then click Save As to ensure that you save that copy of it. It is recommended that you do not overwrite the current version you were working on until you are sure that it contains the right information.

AutoSave

If AutoRecover is not available, you will not be able to recover your file. However you can turn on AutoSave for future use.

To turn on AutoSave, click on File, Options, Save, and then check the box to Save AutoRecover information every xx minutes. Set your desired value (somewhere between 5-10 minutes) and ensure that the box is checked for "Keep the last autosaved version if I close without saving."

Recover Overwritten File

Additionally, you can recover a previous version you have already saved over. You must have AutoSave enabled for this to work.

To recover a previous version, click on File, Info, and next to Manage Versions, select the version you are looking for. It will open up the older version for you to compare to the current file.

Note that previous versions are no longer available once a spreadsheet is closed.