What happened to the old hard drive?
The unique hardware ID is stored in the cloud and should auto activate when connected to the internet.
However for awhile now they have been tied to a Microsoft account, try logging into the same microsoft account if had connected it on your old hard drive - it should activate.
Failing that you can use your windows 7 or 8.x key to re-activate it however if you have already use these - best contact support - link below.
Click get started and type in a brief description and you will be various options to contact Microsoft including a phone call option. support.microsoft.com/en-us/contactus/
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