Can you see any of the numbers? If so, you should be able to contact Microsoft and explain the situation...they're not always helpful, but it's worth a try.
I don't suppose it's on the back of the card? This is what I found: http://funadvice.com/r/3kc9oji6q3
okay thanks, do you know the microsoft uited kingdom Number? The websites useless
thanks for all your help, i have managed to guess the last 2 numbers