If you have the win 10 installation cd, you can reinstall windows just by putting it in the disc tray before you turn on your pc (it will uninstall and reinstall windows and guide you through it)
There's also other options (of course)
1. Go to the desktop and click on the settings icon
2. Select the update and security option.
3. Choose recovery in the left pane and click on "Get started"
4. Choose between keep my files or remove everything.
5. Choose between just remove my files or remove files and clean the drives (I think you should just choose "just remove my files), If you want to be certain that everything is deleted (Choose the other option) it can easily take several hours.Then click on next, Continue and at last RESET. It should fix windows.
If your laptop comes with win 10 pre-installed (And no installation disc) there's almost always a restore to factory setting option but it differs from which manufactor you bought it from.
If it's Hp i think it's F11 on your keyboard, Do this : turn on your pc and keep tapping F11 while it boots. Instead of going into windows it should take you directly to the recovery screen in the boot menu (There you will find the factory reset option).
If your laptop is from Acer i think it's F8, Just do as mentioned above.