Excel has an auto-save feature. I think it is 10 minutes by default. Turn on "Show Hidden Folders". Search your computer for all files ~*.xls and you should find it. When you click on it to open, Excel will probably give you some kind of error or notification message.
If you're using Excel 2010, the last file it saved should show up on the right when you launch Excel and ask you if you want to recover it.
Here is where Excel 2010 on Windows 7 auto-saves files: C:\Users\yourloginname\AppData\Roaming\Microsoft\E xcel\
AppData is a hidden file so you will have to enable viewing hidden files if you try to go after it that way. I would try opening Excel first and see if it prompts you to recover the document.