Unable to start nginx: Not starting nginx as it is disabled in config

Applicable to:

Nginx is stopped:

# service nginx status
nginx is stopped

OR

# service nginx status
nginx dead but pid file exists
Cannot start Nginx:

# /etc/init.d/nginx start
Not starting nginx as it is disabled in config
nginx service is disabled in:
- CentOS/RHEL:
  
  # cat /etc/sysconfig/nginx
  NGINX_ENABLED=no
- Debian/Ubuntu:
  
  # cat /etc/default/nginx
  NGINX_ENABLED=no
Websites are down, or display Apache/Plesk Default Page:

CONFIG_TEXT: Problem loading page

OR:

CONFIG_TEXT: Apache default page
Webmail is not accessible on port 80 but works on port 7080:

CONFIG_TEXT: ERR: Connection refused

Nginx is disabled for starting in it's configuration

In case LiteSpeed Web Server is not used

Connect to the server via SSH
Enable nginx in it's configuration file:
- CentOS/RHEL:
  
  # vi /etc/sysconfig/nginx
- Debian/Ubuntu:
  
  # vi /etc/default/nginx
Set this:

CONFIG_TEXT: NGINX_ENABLED=yes

in:
- CentOS/RHEL:
  
  # cat /etc/sysconfig/nginx
  NGINX_ENABLED=yes
- Debian/Ubuntu:
  
  # cat /etc/default/nginx
  NGINX_ENABLED=yes
Reconfigure nginx and apache to listen to correct ports:

# plesk sbin nginxmng -d
# plesk sbin nginxmng -e
Start nginx:

# service nginx start

In case LiteSpeed Web Server is used

Connect to the server via SSH
Disable nginx in Plesk to create correct configuration:

# plesk sbin nginxmng -d

Note: LiteSpeed requires nginx to be disabled, as it cannot work with it.