How long have you had the set for and when did the problem occur?
Also, what is connected to the TV for devices (Cable Box, DVD, Game system, etc.), and do you get NO sound on the TV regardless of which INPUT you select?
First, make sure the Volume is turned up on BOTH the Cable Box AND the TV. Each has its own independent volume control and if one or the other is turned down, then you will have no sound.
Beyond this, please reset the system by following the steps below:
Please shut the TV and cable box down. Unplug the Cable box from AC power for minutes. During this time, disconnect and reconnect the HDMI cable on both ends, making sure it is secure. Then plug the cable box back in, turn on and let it reboot. Once it's rebooted, then turn on the TV and attempt to watch a program.
The sound should be back on at this point.
If you need further assistance, let me know.