Cannot SSH into server.
Connection refused or permission denied when attempting SSH.
SSH service not running, SSH configuration incorrect, public key not in authorized_keys, or firewall blocking.
Steps:
1. Verify SSH service running: `sudo systemctl status ssh` 2. Check SSH config: `sudo cat /etc/ssh/sshd_config` 3. Ensure your public key is in `~/.ssh/authorized_keys` 4. Check firewall: `sudo ufw status`
Test SSH configuration changes before applying. Keep backup of working SSH config.