One of my servers will never let me on! It gives me an error and I don't know how to fix it any suggestions on how to fix this problem?
The problem is most likely caused by the server, not you.
'Disconnected by Server' means that the server disconnected you (Believe it or not).
It's like asking if you can go to a party. This is equal to 'connecting to the server'.
Your mum or dad then says no. This is equal to you getting disconnected from the server.
In both cases, you can't change the outcome of what happens. The decision of being disconnected is made by your parents not by yourself. Thus, you can't fix the problem on your end. The best you can do is tell the server that you are having problems so they can attempt to fix it.
P.S. There are cases where it is a problem on the client side but the chances of that are quite unlikely, especially if you were able to join the server before. Error messages are still helpful though.