Sometimes when installing some packages or modifying a current theme, the Unity launcher and Dash disappear unexpectedly, trying to reboot the system doesn't solve this. After searching for solutions in the Internet, I have found out that reinstalling Unity is the better solution that would restore both Unity launcher and dash.
Press Ctrl+Alt+T to bring up the terminal and run the following command to reinstall Unity:
sudo apt-get install --reinstall unity && sudo reboot
You can also reinstall Unity with this command:
sudo aptitude reinstall unity