Product key is rejected when installing Windows 10 after replacing the hard drive

OK, you have Windows 10 installation Media, you need to boot your System from that media

On a Dell, just as your system starts, when you see the Dell logo, repeatedly press the F12 key to bring up the temporary boot menu and select the drive with the installation media

Since you previously had Windows 10 installed and activated on that PC during the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 10 on this PC', and activation will not be an issue, your PC will have a digital entitlement stored on the Microsoft Activation Servers

When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key. 

Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate. You will prompted to enter a product key a couple times through the installation, click I don't have a key and Do this later.

There is no need to know or get a new key, but if you have issues with Digital Licensing or the Activation Troubleshooter, you can utilize an existing Windows 7 or Windows 8/8.1 product key you already ownto reactivate if necessary. or use the reset function in Windows 10.

How to find your key and use it to reactivated: 

Click Start > Settings > Update & security > Activation > Change the product key then enter the new product key then click Next to Activate

If you are unable to activate Windows 10 after all the above and you are certain you have the correct edition installed, then please attempt phone activation:

Activate Your Windows 10 License via Microsoft Chat Support

Windows 7 came preinstalled on my computer, how do I find the product key?

The product key is usually located on the COA sticker attached to the bottom of your laptop or in the battery compartment. In some rare cases it can also be found in the memory compartment. For tower systems, look at the top or the side of the system unit. It looks like the following: