Sometimes I miss the Network Connection icon on my top-right panel.This is annoying if you get in this trouble too.Here is an easy way I just found to fix this problem.

Open up a terminal window (Applications->Accessories->Terminal) to run commands.

First,navigate to /etc/NetworkManager/ directory by this command:

cd /etc/NetworkManager/

Then,edit nm-system-settings.conf file:

sudo gedit nm-system-settings.conf

change “managed=false” into “managed=true” and save the file.

Finally,restart the network:

/etc/init.d/networking restart

Done.