As noted on Mojang's site the Minecraft session servers are currently offline preventing players from logging in to any Minecraft server worldwide. We're tracking the status of the issue and will update this post as new information becomes available.

Please refrain from contacting ticket or live support about this issue - it is out of our control and we're trying to help as many users as possible.

Update: The session server issues should now be resolved. If you are still having issues, log out of Minecraft completely by closing the game, and then log back in and the problem should be resolved.