The following points should be checked:
Make sure that port 8443 is opened on the server and listened:
# nmap -p8443 -P0 server_IP
# netstat -nap | grep 8443
- Check panel logs:
# less /usr/local/psa/admin/logs/httpsd_access_log
# less /usr/local/psa/admin/logs/httpsd_error_log
Also, you can check main system log:
# less /var/log/messages
Did you see any error during Plesk start?
# /etc/init.d/psa restart