Open the terminal (CTRL+ALT+T) and stop the network manager process if it is still running in the background with this command:

```sudo service network-manager stop```

Now remove this file using the following command:

```sudo rm /var/lib/NetworkManager/NetworkManager.state```

Start the network manager now with this command:

```sudo service network-manager start```

The network manager icon will appear again in the top panel of Ubuntu Unity.