Did you purchase a Windows 8 Upgrade or Full Edition? If an upgrade, then you will need to re-install Windows 7 using the product key on the sticker and then upgrade to Windows 8. If you purchased the full version of Windows 8, then you can just use the product key that came with that purchase.
Here is a link to the various Windows 7 ISOs
for re-installing Windows 7. This plus your product key should get you back in business. Just be sure to download the version of Windows 7 that was initially installed (Starter, Home Premium, Pro, Ultimate and 32- or 64-bit).