It's because the session server is down.
When you cannot login to minecraft or a server, check here:
http://status.spout.org/If the login server is down, then you cannot login to minecraft.
Solution: If you have mods inside your minecraft.jar, copy it onto your desktop and get a temporary minecraft.jar and paste it in your bin folder then play from the browser (minecraft.net/play)
If you do not have mods inside your jar then you can play straight from the browser.
If the session server is down, it affects multiplayer and is the reason why you cannot login to the server right now.
Solution: There is no solution to this, besides playing single player or taking a break from minecraft until its back up.