Problem/Bug Server won't start

So, on my Mac, I have a server. It works fine until like 10:00 or 11:00. Now, every time I try to start my server. this happens in the terminal

Code:

[11:43:40 INFO]: Starting minecraft server version 1.12.2
[11:43:40 INFO]: Loading properties
[11:43:40 INFO]: Default game type: CREATIVE
[11:43:40 INFO]: Generating keypair
[11:43:40 INFO]: Starting Minecraft server on [IP HIDDEN FOR SECURITY REASONS]
[11:43:40 INFO]: Using default channel type
[11:43:40 WARN]: **** FAILED TO BIND TO PORT!
[11:43:40 WARN]: The exception was: java.net.BindException: Address already in use
[11:43:40 WARN]: Perhaps a server is already running on that port?
[11:43:40 INFO]: Stopping server
>logout
Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.

[Process completed]

Anyone know why this happened, and how to fix it?

 

Simpler version of what Tim said:
Close down any servers which are open and re-try starting the server

- ES Team.