Common causes for this include:
Another DJ or Auto DJ is already connected.
SHOUTcast can only have a single DJ connected at any one time. You can check if another DJ is connected by going to the overview page of your account and clicking the link directly under "Full Server Address" to access your server DNAS status page.
A DJ is connected
No DJ connected
If our Auto DJ is connected, you can click "Auto DJ" from the menu on the left and then "Stop Auto DJ" to disconnect our Auto DJ.
The connection to the server is blocked by a firewall or filter
Many school or workplace networks block or filter traffic to non-standard ports, if you are on such a network, provide your IT administrator with the address and port of your server and ask if it's been blocked.