Hi Krishna,
If your Lenovo had Windows activated before, you do not need a product key. Microsoft will not issue a new key for an OEM device because activation is done through a digital license tied to your hardware or to your Microsoft account.
Do this to reactivate without a key. Connect the PC to the internet and sign in with the same Microsoft account you used before the reinstall. Open Settings > System > Activation.
If it shows Not activated, select Troubleshoot. Choose I changed hardware on this device recently. Sign in if asked, pick your PC from the list, check This is the device I am using right now, then select Activate.
If it still does not activate, tell me exactly what you see in Settings > System > Activation, the Windows edition shown there like Home or Pro, and any error code. I will guide you to the next step.