If you are receiving a “Can not connect to server” prompt, the best course of action to take would be to completely close the client, relaunch it, and try again.
If you are able to see a “logging in” prompt and it fades, please be patient as at this point, the client is attempting to establish a connection and/or will place your account in a temporary ‘ghost’ queue, where you will place into the server at a later time without being able to see the queue box or the number you as placed in the queue.