You may need to determine which message is displaying. Which one is it?

http://www.google.com/search?q=OPERATING+SYSTEM+NOT+FOUND finds many prior discussions for you to decide what is similar to your issue.

Please report if you are not seeing the forum tip which reads -> Tip: If you are asking for help to troubleshoot a computer-related problem, please be sure to include all the necessary information (ie: operating system, model number, hardware, software, etc) that will help others identify your problem for a speedy resolution.