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.
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