Minecraft can only play in offline mode?

Every time I try to play Minecraft, the launcher screen is blank and it says 'play offline' instead of play. I don't know whether its the actual Mojang servers or my game, but I've uninstalled minecraft and re installed it, and it still says the same. I've logged out but i can't log back in because it says: 'Sorry, but we couldn't connect to our servers. Please make sure you are online and Minecraft isn't blocked.' Does anyone know what this means?