Keys can be blocked for a variety of reasons; installed on more than one device, pirated, etc.
Blocked keys is a problem for Microsoft to explain to you.
You contact them back again and tell them from us that they are wrong. They have to help you with activation.
We don't work for Microsoft and Microsoft Agents never look at these Windows 7 forums.
We can't activate it for you or explain why it is blocked.
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We can only give you Microsoft's advice about Activation problems, not BLOCKED Product Keys:
How to See Your Windows 7 Licence Information
https://www.sevenforums.com/tutorials/19921-license-information-windows-7-a.html
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List of Activation error codes and what to do about them:
Error message when you try to activate Windows 7 or Windows Vista: "An error has occurred"
Contact Microsoft:
The COA sticker, which has the Product Key on it, is either on the underside of the Laptop or in the Battery Compartment.
With a Desktop it is either on the side, rear or top of the Computer Case.

Activate Windows 7 or Windows 8.1 - Windows Help - Microsoft Support
When you begin activation, Windows will try to use your Internet connection to activate online. If you have problems activating online, you can activate using the phone.
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If you have to use Phone Activation, read this detailed information about using that method:
"How to Activate Windows 7 by Phone"
http://www.sevenforums.com/tutorials/18715-activate-windows-7-phone.html
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If you have problems using that method contact Microsoft Support:
To contact Microsoft Support:
Contact support
http://windows.microsoft.com/en-us/windows/contact-support