The error io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused appears when Minecraft tries to join a server and nothing answers at the address and port given. It means the connection was actively refused, not lost on the way, so the usual causes are a server that is not running, a wrong address or port, or a firewall blocking the connection.
The fix, in order
Confirm the server is actually running. If it is a personal server, check that the server program is open and has finished starting. If it belongs to someone else, ask whether it is online.
Check the address and port. Confirm the server address and port number with the server owner, and type them exactly. The default Minecraft port is 25565, and a server on a different port needs the port in the address.
Try the server address without a port first, then with the port, since a wrong port is a common cause of a refused connection.
Check the firewall on the server PC. The server port must be allowed through the firewall, or outside players cannot connect at all.
Restart the router and the server PC, then try again. A stale network state can refuse connections that should work.
Check that the game version matches the server version. A version mismatch gives a different message, but it is worth ruling out while troubleshooting.
Test from the server PC itself. If the server works locally but not from another PC, the problem is the network or firewall, not the server.
If it still happens
If the server is hosted by a hosting company, check the host's status page and control panel, since the server may be offline or suspended. If the error only happens on one network, such as a school or workplace network, that network may block game traffic, so try a different network to confirm.
Common questions
Does connection refused mean I am banned? No. A ban shows a different message, usually naming the ban. Connection refused means nothing answered at that address, so the server is off, the address is wrong, or a firewall is blocking it.
Why can my friend join but I cannot? The server is running, so the problem is on the route from this PC to the server. Check the address and port, the firewall, and whether a VPN or a restricted network is in the way.