Why the Server Isn't Updated:Due to the new UUID system in 1.7.7 in preparation for username changes coming in 1.8, there has been many issues caused because of this. The way plugins were coded before, will now have to somewhat be recoded for this UUID system. Until bukkit and all the core plugins are updated, the server will not be updated to the latest version. But wait! There's some good news! You can still play on the server. Yes, you read that correctly! The server is still compatible with versions 1.7.2 through 1.7.5.
How to Go Back to Version 1.7.5:
Open your minecraft launcher and locate the "Edit Profile" button. Click on it and find where it says "Use Version:". Once you have done that change the version to "release 1.7.5" and click save. After that you will click "Play" and you will be able to connect to the server once again. Once the server is updated, you will need to repeat this process but select "Use Latest Version" instead of "release 1.7.5". If you have any trouble, feel free to comment.
Once the server is updated, I will update this post.