Login to your Microsoft account and see what is registered there, and if you can transfer it.
That's possible if the original Windows 7 on your old PC was a retail version, not an OEM version. Since you didn't tell the version, we can't tell. But it won't harm to login and see.
the title says most of it but I will explain.
so I was building a new custom pc build and decided to transfer my digital key I got on my other old computer that I upgraded from windows 7 from to my new one, therefore not needing top buy windows again. so I thought I had the product key written in my phone notes and typed into cmd slmgr/upk and now I don't know what my key is.
if someone could help me out or just give me the definite news that it;'s gone forever that would be great, thanks!!

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