NOTE: These will only work if you can get into the system. They will not work if you can't even get into Windows.
In Windows versions prior to 10, you should be able to...
1. Go into "Control Panel" and then click "Recovery."
2. Next click "Advanced Recovery Methods."
3. Now click "Return your computer to factory condition."
And in 10...
To do this go to "Start", then "Settings". Next choose "Update & security" and then "Recovery" Here click "Get started" and select the option you wish to use. From here just follow the on-screen instructions to restore Windows 10 back to how it was upon leaving the factory.
If none of this works for you either, then you may well need to get your hands on a CD/DVD/USB with the original system on it and do the reinstall that way.