First: take a full system-image backup as soon as possible, and use that as a fallback. You can always restore to that image in lieu of a reinstall, and you’ll be back to where you were at the time the backup was taken.
You could also get in touch with the vendor who sold you the computer. They provided you with a copy once; perhaps they’ll be willing to get you a replacement copy.
You could, I suppose, go buy a new copy. Most people aren’t interested in doing that, because they don’t want to pay for something they feel they’ve already purchased.
That’s when most people resort to finding a place from which to download Windows. Depending on the version of Windows and where you find it, it might even be legal.
Or it might not.