Dell Vostro 3900 windows 8.1 product key

Hi,

I bought a new Dell Vostro 3900 with Windows 8.1 pro pre-installed last month from Dell. I tried to update to windows 10 but the software crashed my pc. 

I managed to boot up in windows 8.1 again but is now asking for a product key. I cant see one on the pc case and never had an email or any paper work with a product key on it.

\Is there anyway to find out what my product key was, so I can re-enter it to activate windows again?

Thanks in advance,

Stuart