On another computer, download Windows 10, for free, from: Download Windows 10 (microsoft.com)
Either write the download to an empty 8GB (or larger) USB memory-stick, or burn the image to one DVD-writable disk.
Boot your computer from the above media, and choose "Repair" instead of "Install".
Since you were running Windows 10 on your computer, you can click "I do not have a product key", if you see that prompt.
The installer will "call home", and download your previous product-key, and re-activate Windows.