The netstat is a good starting point to find out if something is using port 80. But judging from the icon below the OP's username he's using a Debian based distro (Ubuntu). Hence the configuration files are different:
Global configuration = /etc/apache2/apache2.conf
Listen directives = /etc/apache2/ports.conf
(if I'm not mistaking).
Restarting should be on Debian based: