Well, the bad thing about Windows 10 auto-upgrade is it MIGHT delete some recovery partitions or at least misalign them (like Lenovo OKR, it has REALLY specific instructions for its recovery partition, like it should have 0x10 (not the real one, I'm just throwing that ID off my head but google it and you should find) partition ID, must be the fourth partition in the drive etc.) and as MBR supports only four partitions per drive, there is a good chance it's unusable. You could bring it back, but it's A) not worth it, B) will require at least two full wipes. Though the instructions differ, you'll certainly have to do a wipe.
So, what will I recommend is, if it hasn't been 30 days since the upgrade happened, you can roll back.
"For a limited time after upgrading to Windows 10, you’ll be able to go back to your previous version of Windows by selecting the
Start button, then select
Settings >
Update & Security >
Recovery and then selecting
Get started under
Go back to the previous version of Windows 10. This will keep your personal files, but it'll remove apps and drivers installed after the upgrade, as well as any changes you made to settings. In most cases, you'll have 10 days to go back. "
Or, if it has been 30 days, you will need to reinstall the whole Windows 7 using an install media. You will have to do a fresh install, you can pretty much copy off/image whatever is important using Macrium Reflect (imaging software) or Hiren's Boot CD (recovery CD) and reinstall or copy them back when you're there.
As far as I know you can still retrieve the key from the Windows 10 install and reuse it to activate your Win 7, but you'll have to do the phone activation. Don't take my word for it though.
https://answers.microsoft.com/en-us...-30-days/9240f20c-38ce-4bcb-86e5-da6c3d768ccd