Please open the terminal from Applicaions->Terminal and input the commands that Pavol gave in his last post then tell us what happens.
One other thing, make sure under the wired connection, you check the box that says start network automatically. If that box is not checked it may be the reason for network manager not starting. Although as said in a previous post by Pavol:
sudo service network-manager restart
Just some additional procedures, unplug your switch and plug it back. Then check that your router is working properly, like receiving activities, this means it is connected to your ISP. If not, restart your router by powering it off and on. Once you've done that, apply the procedures in your terminal as stated by the previous posts.
To input the code, open your terminal from applications>accessories>terminal
Then in the terminal, type as follow
#sudo service network-manager restart
The system will ask for your password, and once given correctly, your ethernet device will be restarted.