The reason why the OS may have to be re installed was that you downloaded some sort of file with a vrius which has infected the system or your HDD or SSD is not working properly. So when you get your OS running again, get some anti virus running because this may be the problem. You can pay £40.00 for ESET but you can use AVAST! which is free for a few days.
Reply me back if this isn't the problem.