Restart your laptop. constantly press F8 when the company's logo that made the laptop appears. You will then be given the option to change back to factory settings
I cant give you a much detailed tutorial as Im not sure what laptop yours is. A box will appear with numerous options. Try all of them
This will erase everything on the laptop and leave behind all the things that came with the laptop with no OS disc. It may take a while, however, to reset