1. Use Notebook to directly test the socket of switch. Try to get a dhcp address if the network system is ok.
2. Sometime the host is strange in the network connection. Try to restart the network interface (e.g. service network restart, ifdown eth0; ifup eth0...). Or we can try to restart the OS
3. Try to change to use another network line, another pc to test the network line, another switch/hub to distinguish the real point of network failure.
My experience of facing the network disconnected in the company :
Find the network root line, e.g. if the lab is network disconnected, we can test the network root line directly. Sometimes the real problem is that the foreign network device was down (e.g. switch, router) or the foreign network line/socket was not connected well causing the network unstable.