I'm guessing your hard drive died, so if you want to fix it, figure out what kind of hard drive will fit in your laptop and work, and then install whatever operating system you want on it, so if it's a prebuilt laptop, I think you're probably going to have to buy a copy of Windows if you want Windows and not Gentoo.

If it's any kind of half-modern laptop, you want a 2.5" SATA hard drive or SSD.

You should steer away from the few that are 12.5mm thick, the thinner ones will always fit.

If it came with Vista or higher, then it's likely the license key is in the BIOS and you just have to reinstall. Unless you want Linux and safely ignore any license bullshit.