The most common reason you see a message like that is because you've been playing on a server that has crashed and thus when you attempt to connect to another server the CD key will be considered to be in use.
Solution to that problem would be to reconnect and log out from the server you crashed in (when it pops up) or you simply wait (5 minute-ish or so).
Speculation on a less common reason would be that your CD key is still somehow connected to the server even though you have logged out, a potentially easy solution would be to IP release/renew or it might even be so that you have to wait for the server to reset.
The last and least common reason would be that someone else uses the same CD key you do.