Several incompatible updates to the 1.7 branch of Minecraft have been released over the course of the last two months, rendering clients that have updated unable to connect to our server. We're still running 1.7.5, so if you want to connect, you'll need to create a separate Minecraft profile with the appropriate version selected.
Currently, the only Bukkit releases that support up to 1.7.9 are handled by their development branch, which we do not update to. Thus, we'll be holding off on the upgrade until a beta is released.
Be aware that 1.8 will be out soon which will (again) substantially change the terrain generation, so we may have to regenerate the world. Again.