1) Put a Windows 8 Disc (it doesn't have to be a genioune one or the one you used to install that one OS, just some windows 8 disc)
2) Go to repair options
3) Go to advanced options
4) Open the command promt
5) Type in " bcdboot c:\windows /s c: " and press enter (if you have windows installed on other driver other than C: type in that one driver letter instead of c and the rest is the same)
Now you can type "exit" to exit and enter and then you can choose to shutdown the computer in the further options
Shut down the computer and it will now boot normally.