Why does the game keep....?

Asking me for my serial number. the game crashed while I was playing it, now every time I start the programme it asks me to enter my serial number but when I do it says "your serial number has been blocked" it also says my online serial number is invalid. At the moment I am completely unable to play the game. Note: I have been playing the game and this has only just started to occur after the game crashed. I have tried uninstalling and reinstalling the game and nothing seems to work. I bought and downloaded this game for pc from game.co.uk.
Does anybody have any suggestions what I can do?Thanks.