Ok if you didn't save the changes and the UNDO option is not working then I am afraid you will need to open your last saved version and work from that.
Have a look at this article which will teach you how to use autosave in the future to prevent this from happening again. http://office.microsoft.com/en-us/help/automatically-save-and-recover-office-files-HP010140729.aspx
Here are quick instructions on configuring:
1) While Excel 2007 is open, go to the top left corner and click on the MS Office ribbon button
2) Click on the Excel Options button to the lower right corner of the window (this option is to the immediate left of Exit Excel
3) With the Excel Options window, look in the left for pane for the Save option
4) Under Save Workbooks, select the Save AutoRecover information every _ minutes where you select the time interval Excel should be saving AutoRecover info for such instances.
If you do find that you had autosave configured look for the path to where the autosaves are kept within the setup window and you can try to recover your work that way.
If you need further help please click "reply to expert" to let me know.
If you are satisfied with my answer please click accept.