There are several possibilities.
• firewall, iptables configuration
• apache listen address / port

If the issue is with the firewall/iptables, you can add the following lines to /etc/sysconfig/iptables:
-A INPUT -p tcp -m tcp –dport 80 -j ACCEPT
-A INPUT -p tcp -m tcp –dport 443 -j ACCEPT

(Second line is only needed for https)

Make sure this is above any lines that would globally restrict access, like the following:
-A INPUT -j REJECT –reject-with icmp-host-prohibited

And finally

# service iptables restart