Cannot connect to control panel..

Services work, but control panel won't load when trying to access the web panel.

This happened out of nowhere.. It was working fine one day, next day I try to login and it won't load. Tried restarting the services and the server, still the same issue.

 If I have to reset the entire server and reinstall the panel every time a weird bug like this happens all alone, it will impossible to use this panel.

It is the firewall, all the instructions are in the panel as well as the wiki pages. Could be mod security too, could be a dns issue. If all else fails and you can not get help then as a last resort backup account and reinstall the server. When there is an error there is usually logs and you can fix the errors from reading the error logs for the webserver and other services.