Windows 8 not booting

I have a ASPIRE ONE 756 laptop that comes pre-installed with windows 8. The laptop worked fine but recently crashed. When i power on the laptop i get the error message on POST ” Windows Boot manager has been blocked by the current security policy” I have restored BIOS settings to default but its not working. IF i have to re-install windows where will i get the key from as i have noticed with windows 8 there is no key at the bottom. Any help will be appreciated.