@krishna Using a random screenshot and not explain it costed you two days by pointing to the wrong direction. The clearer your information is the more we are able to help you.
OK, so this is a normal single master FOG node setup and we can rule out routing because FOG server and PXE booting clients are all in the same subnet?
So you have the firewall disabled in the FOG server?
Where clients ever able to PXE boot to the menu? If yes, when did it work last and what changed since then?
About the access.log - I just figured that Ubuntu moved to a different name at some point. Please run tail -f /var/log/apache2/*.log while you PXE boot one client to see if it shows up in the log.