The game client was unable to connect to the log-in server.
You should be able to log in now.
EDIT: The game is up and running again. (That was pretty fast.)
This message also shows when the Guild Wars servers themselves are down (e.g. for maintenance); I would suggest to try again later.
Right now, I can confirm that the servers are down for maintenance, there was an in-game message. The official GW2 wiki seems to be affected, too.
I think it should take less than an hour, but that is just me guessing.
Ok, thank you