Reloaded computer and I need my product key for my pre-installed full version of Microsoft Office.

I restored my Dell Inspiron with Windows 7 system to factory condition using the DataSafe program.  My pre-installed full version of Microsoft Office that came with my original system is requesting the product key, but I do not know if I ever had a product key.  My computer is past its warranty time.  How can I get the code?  Thanks.