By far the most common cause of this problem, is a glitch in the cable box processing.
Please follow the steps below:
1) Shut the TV off, and then disconnect the cable/SAT box AND the TV from power (unplug their power cords).
2) If using an HDMI cable, disconnect and reconnect the HDMI cable on both ends when the cable box is unplugged from power (make sure it's secure on both ends).
3) Press and hold the power button on the TV itself (not the remote) for 10 full seconds while the set is unplugged, then release the power button.
4) Plug the cable box AND the TV back into power, then turn the TV on and attempt to watch a program.
This should restore the sound. However if it does not, then please try another source- such as a DVD player on the TV and see if that has sound. This tells us whether or not the TV, or the cable/SAT box itself is causing the problem.