Do you have any third-party repositories enabled? You can manage them from a GUI with the “Software” manager application. Click on the menu button (upper-right corner, three horizontal lines stacked vertically) and use the “Software Repositories” item.
It sounded like you were able to update your system recently so it’s odd why it’s not working now.
You could update your /etc/apt/sources.list manually as well. Just follow steps 4-8.