This happens to me every now and then. What causes it most of the time in my experience is plugging something into your computer that changes the jack to auxiliary. What usually works for me is this:
1. Go to "System Preferences." If you don't know how, you can either look it up in your Spotlight(CMD+Space) or go up to the Apple logo in the top left of your screen.
2. In the 2nd row, "Hardware," click "Sound."
3. If the option "Aggregate Device" is highlighted, highlight the option "Internal Speakers;" if you have headphones or external speakers plugged in, choose those.
I hope this helps! :)