You can install Caffeine to do this:

sudo add-apt-repository ppa:caffeine-developers/ppa

sudo apt-get update

sudo apt-get install caffeine

Then search for Caffeine in your Dash to start it. It should start automatically after a reboot.

Now Caffeine will show an indicator in the top right of your screen. Click on it to enable Caffeine (your PC won't go to sleep now). Click again to disable it and Ubuntu will follow its usual sleep schema.

You can also disable the sleep permanently if you prefer.

1. Open System Settings


2. Click on Brightness & Lock


3. At Turn screen off when inactive for select Never