If none of these things are working you might need to stop and restart your network. In Debian, this is /etc/init.d/networking restart, while in SuSE it is /etc/rc.d/network restart and in RedHat the /etc/rc.d/init.d/network restart will suffice.

Troubleshooting the network is really using a combination of tools to determine what the problems are and then going from there, knowing where to go and look for the solution to those problems, using the configuration file tools to your disposal.