You close Excel and answer "Don't Save" when you really needed to save. There are two potential ways to get the file back.
Thanks to Office MVP Beth Melton for rounding out my knowledge of this trick.
Workbooks that never had a name
You have a new workbook. You make changes. The workbook gets through an AutoSave
You close without saving.
When you open Excel, scroll all the way down to find Recover Unsaved Workbooks
Workbooks that did have a name
You open a workbook. You make changes. The workbook gets through an AutoSave
You close without saving.
Normally, Excel keeps last five autosave versions (one every 10 minutes, provided there has been a change)
When you close without saving, all are lost, except the last version.
Go to File, Info...