This can be caused by an U.S.G.N. overload and other problems.

Maybe the U.S.G.N. is down. Check its status at USGN.de and try again later

Open the console to get detailed information about the problem

Make sure that your PC is up to date. Run windows update and reboot

Join a server without serverlist: Click at "Connect to IP" and paste the ip of the server
You can see servers but the login does not work? Please read
"I can't login! Failed to login!"!