Somebody probably hacked your key just by chance, you'd prolly want to go to the MFGR site and request a new key, I dunno about CoD, but for Half Life 2, they want a picture of your box, CD, and Receipt.
Friend of mine gets this the first time he tries to connect to any server, usually works second time round though. Caused by his shitty ass cheap router, if it's got a firewall built in, manually set the port forwarding for it, or turn it off if you're lazy/brave/stupid.