"Was working as genuine", do you not know if it was genuine? How long ago did you buy it and from where? Did W7 come with the computer or did you buy a key off the internet?
Product keys can work fine for 3 or 4 months but if they are being used by multiple people then Microsoft takes notice and marks all of them as invalid; that is why it is not safe to buy product keys.
If there is a license key on the computer then enter it and try to validate again. Or call Microsoft.