No Multi-player Servers Allowed

You load up Minecraft, and the button/sign (Where it says "Play" at the bottom) says "Play Offline". I do know it's not my Wi-fi. But before I logged out of the server I was in, it was working perfectly fine. Now it won't let me log-into any servers. It says, "Failed to login: The server encountered an unexpected condition which prevented it from fulfilling the request." It'd be nice if you could fix this problem as soon as possible. Thanks!