Hi Shivam. I'm Greg, an installation specialist, 10 year Windows MVP, and Volunteer Guardian Moderator here to help you.
How was Windows 10 originally Activated on the PC? Did it come preinstalled from Factory, was it Upgraded with free or paid Upgrade, or was it a retail copy and license?
For Upgrade, when you first upgraded and it activated it assigned a permanent Digital License to the PC in Microsoft Activation Servers so you can reinstall any time and in any way you want as long as you reinstall the licensed version. It should reactivate soon after going online. More about Digital LIcenses vs. Product Keys here:
https://support.microsoft.com/en-us/help/12440/...
You can move the Digital License to new hardware following this procedure: https://support.microsoft.com/en-us/help/20530/...
Factory installed WIndows 10 has the Product Key embedded in BIOS or UEFI firmware and should activate itself, but you can check for the key by installing ProducKey: http://www.nirsoft.net/utils/produkey-x64.zip to copy and paste the OEM-BIOS key into Settings > Update & Security > Activation > Change Product Key.
Retail keys must be reinserted for reinstalls during install or in Settings.
What is the Status and Edition shown at the top of the page at Settings > Update & Security > Activation?
Run the Activation Troubleshooter at Settings > Update & Security > Activation. If it says you have a Digital License then no matter what method above only the Generic Version Key can be inserted to reactivate: http://winaero.com/blog/generic-key-to-install-...
If this fails you can contact Microsoft Support: https://partner.support.services.microsoft.com/...
I hope this helps. Feel free to ask back any questions and keep me posted. If you'll wait to rate whether my post helped you, I will keep working with you until it's resolved.
________________________________________________________
Standard Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.