I think you've misinterpreted what you want to do. Rebooting doesn't require a product code (key you mean)? If you want to retrieve your original installation key, simply use a program like WinKey Finder, or Magic JellyBean.
^^^ I agree. Magic Jellybean finder works great. Nice and simple. That is, if you can get into Windows. If not, then you are out of luck. You will need to contact Microsoft and explain the situation to them. You may have to purchase another license. Good luck, ~Rich