oh no! the key should be on the packaging the disc came on? if you bought it online, go to this sitehttp://support.microsoft.com/gp/esd-support-phone-numbers
you will have to give them a call, with your details & the credit card you bought it on & they will send you the details.
I think you can download the iso from microsoft's website. also i think windows 8.1 is tied to your microsoft account, so if you log into the ms store with your computers sign in, you may find it there.