--[option 1]--
extract the physical drive.
use free recovery software.
pin point, and recover the product key.
restore OS with windows OS disc/usb image.
apply product key and get back on your feet.
--[option 2]--
if it's windows 8 and you think the product key is not readable, the key is stored in the motherboard. install a trial key of 8.
acquire a BIOS key extractor here:
http://www.sherr.co.uk/Distribution/Software/W8key/WpKey.zip
the utility will attempt to extract the product key from the BIOS.
--[option 3]--
contact the hardware manufacturer for recovery media. more times than not, the supplied DVD's will automatically activate Windows for end users attempting to revive their OS with the help of manufacturer supplied recovery media.
--[option 4]--
purchase a discount key from a 3rd party (much more risky and not guaranteed).
--[option 5]--
anything and everything else anyone has mentioned here.