Replaced hard drive

On my laptop, I had to replace the hard drive. It wouldn’t boot, gave me a disk error during boot. Went into BIOS, did HD check, and it failed. Now, how do i get a copy of windows to put on the system? HP doesn’t give you any discs to reload the OS. Do I have to buy another copy, or is there a resource that would allow me to download and use my product key?