A Windows Product Key is five blocks of five characters but if you reinstall the same version of Windows into the same laptop, the system will automatically remember the key and you won't need to reactivate.
You can find the download for your version on Microsoft's website. Burn it to a CD or DVD or put it on a memory stick or USB drive, use Rufus to make that stick bootable.
Many laptops have a factory recovery utility which reinstalls Windows & drivers so your laptop is back how it was when you bought it. I would investigate that option first - - will save you the hassle of having to source & install drivers manually, and factory recovery will definitely not ask you to enter the Windows Product Key.