Hi
Just to clarify things, when you are saying "Reboot" are you talking about restarting your PC or reinstalling the original Operating System?
I'm not clear on that since you don't need a CD, DVD to reboot it.
Is it a Laptop, (which would probably have a restore partition) or a desktop?
If it's a laptop your best bet would be to reset it to the factory defaults.
Save all your data to someplace off the computer before you do that.
What brand of computer is it?
If the operating system was installed by the manufacturer you can probably get the serial number from them by giving them the serial number of the computer.
If you have a legitimate Windows 8 key then it should work, as long as it hasn't been used on another computer.
Is the computer running at the present time?
Or are you not able to start it?
If it is up and running but you can't activate it you could install Belarc Advisor, It may be able to show the original Windows serial number.
http://www.belarc.com/free_download.html
It can detect pretty much everything about your computer including all of the serial numbers for all your software.
Mike