There is no way I could know what kind of setup you have so I'll offer a couple of generic suggestions to diagnose the problem.
1. Go to the main menu, select Audio and scroll to the last option. It should says TV Speaker: ON, if not, then that is your problem. Also, after verifying that the TV Speaker setting is "ON", also select the reset option in the Audio menu (you may want to record your previous Treble and Bass settings so you can restore them later).
2. Check your connections to the back of your television and accompanying devices. See if they are firmly in place. Remove and re-insert them to be certain.
3. Check a different source. If you are using a Cable box for your signal, trying switching to a DVD/blu-ray player and checking if it has sound. That will let you know if the issue is with the TV itself or one of the devices that you are using. You can also try changing the connection port on the back of your TV. So, if you had your device plugged into HDMI 1, try changing it another HDMI port (remember that there is also a port on the left side of the TV you can try).
4. Lastly, try turning off your TV and unplugging it from the electricity port. Wait 5 minutes, plug it back in, and turn it on.
This TV has 2 years warranty from LG I believe so you should call them if you can't get this issue resolved.