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This you call an upgrade? 😀 😀 Start Mint and look in Dolphin or whichever file manage it uses for information on the hard drive's contents. If there's a Windows 7 Recovery Partition you might be able to use that to start the system again.
Failing that, and so long as you have the Microsoft Certificate of Authenticy on the case and the Product Key is legible, it is permissible to install from a borrowed disk of the same level, say Home Premium. I don't allow the system to be online as installation completes in casee there isn't a message to avoid immediate activation but if you see that message, tick to avoid it.
As soon as the system starts for the first time, go into Control Panel>System and click on Change Product Key. Use the key from the CoA then allow the system to go online and check that activation completes.