If it's a laptop with Optical Drive/DVD/CD, you could download a copy of WIndows ISO burn it to DVD disc, it has to be somewhat the same as your Laptop's version OS, which you only need it to repair your Windows OS, if you don't have Optical Drive, which no worries, you can use a USB flash memory to boot up a copy of the window you used. If that doesn't work, just re-install the whole OS with the copy of the windows, and use the product key underneath your laptop. If that doesn't work contact Microsoft via forums, ask them what to do, and how to install fresh copy of windows, without loosing your OEM license to your laptop.