At the moment everyone who is playing with their region settings on USA won't be able to open missions or use the "Return to Haven" buttons. You can still join other Knights in their Missions or in Haven, but those will be running on European servers.
So in order for yourself to be able to do everything you'll have to change your settings to Europa until the USA servers are back up. During this time you'll probably experience some lag issues.
For the players from Europa there shouldn't be any problem, most of them probably won't even realize the downtime of the USA server.
Good luck playing, see you in the clockworks.
Edit: Changed for USA serves.
Hennils said pretty much what I was coming to the forums to say (thank you, Hennils!). My apologies for the inconvenience; we've roused the engineers from their nice warm beds and they're working to resolve the issue.