SSH connection refused or timeout -- systematic debugging

Cannot SSH into a server. Connection refused, times out, or key auth fails.

Debug: (1) Is sshd running? (2) Port 22 open? Check firewall/security groups. (3) ssh -vvv for verbose. (4) Permissions: chmod 700 ~/.ssh, chmod 600 keys. (5) Check ssh-add -l for loaded keys.

SSH failures span network, firewall, daemon config, and permissions. The -vvv flag shows the exact failure point.