On my PC after installing Ubuntu 12.04 my networks are completely gone. Inside the terminal, after typing in nm-tool I get the following:

The program nm-tool is currently not installed. You can install by typing:
sudo apt-get install network-manager

After I type that in then my password I get this:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
  network manager : Depends: iputils-arping but it is not going to be installed
E: Unable to correct problems, you have held broken packages

I'm a complete novice when it comes to computers so I have no clue.