The Minecraft login and session server's are offline, Mojang is doing some database updates to they're servers.
You can check the status of them here:
http://xpaw.ru/mcstatus/
https://help.mojang.com/
UPDATE: Back online!
You can check the status of them here:
http://xpaw.ru/mcstatus/
https://help.mojang.com/
UPDATE: Back online!