Hi,
MS Office 2011 has an "autosave" feature. It makes a backup every 10 minutes, by default.
You should be able to access the saved file by the Excel "File" menu : File / Open recent.
If you don't find it there, you can search the file in your User Library at :
your_account/Library/Application Support/ Microsoft/Office/Office 2011 AutoRecovery
To access the (hidden) User Library : Finder / "Go" menu, press the Alt key to reveal the Library in the list.