I have just come off playing counter-strike, ive had the game 2 year this has never happened to me but after coming off a server and I try and connect again it says someone is using my cdkey? i know counter-strike is full off hackers now but is it possible for them to obtain my cdkey even though I have a firewall running?

Nobody hacked it by accessing you though your firewall. The server probable still thinks you're connected and needs a little time to re-syncronize. Wait a few minutes then try again.

The only way someone could get your CD-Key is if you share it with someone, who shares it with someone else, etc.

There are cd-key generators about though, I'm not sure how good these are though, I also have HL and CS etc and have never had this problem before. However what QWERTY said should work fine.