This means that session servers are offline. Session servers are pinged anytime a player wants to join a server, to obtain a valid lease of identity. This is needed to make sure that for example player nsanidy who just logged in is in fact our nsanidy, and not some hacking shmuck, pretending to be him.
http://xpaw.ru/mcstatus/ ← bookmark this page and check whenever you have trouble logging in or connecting to MCNSA.
As you can see there’s major outage. However, should the website above indicate that Mojang’s servers are OK and you still can’t login, message us.