Regarding loseage of windows 10 product key - Microsoft Community

i had upgraded pc windows 10 @ time of free offer. due hardware defect had changed computer hard disk in local authorized lenovo service centre. in process has installed non-activated version of windows 8.1. due non-activated version, system has lost customizing option , many more features. there way restore windows 10 product key can upgrade windows 10. old hard disk not working.                                    

you never were given a windows 10 product key free upgrade 10.

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find windows 10 media creation tool on software download page 

"start fresh clean installation of windows 10"

https://www.microsoft.com/en-us/software-download/windows10startfresh

"how clean install windows 10"

http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html

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when your free upgrade 10 was  activated microsoft have kept the hardware configuration details of device on servers recognition purposes if have reinstall 10.

"activation in windows 10" (digital entitlement)

https://support.microsoft.com/en-au/help/12440/windows-10-activation

cheers.

Windows / Windows 8.1 / Install, upgrade, & activate / Activate