If you are referring to your Windows licence key, then go to http://windows.microsoft.com/en-GB/windows/help/genuine/product-key and use the Microsoft Product tool which will guide you through the process to purchase a new product licence code.
If you are reinstalling the exact same version of Windows on the exact same computer / hardware where it had previously been activated, then you should be able to contact the Microsoft Activation Centre by telephone and be guided through the process of re-activating your software.
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If you are referring to your Windows licence key, then go to http://windows.microsoft.com/en-GB/windows/help/genuine/product-key and use the Microsoft Product tool which will guide you through the process to purchase a new product licence code.
If you are reinstalling the exact same version of Windows on the exact same computer / hardware where it had previously been activated, then you should be able to contact the Microsoft Activation Centre by telephone and be guided through the process of re-activating your software.