Try this first -- Unplug your Cable box or whatever is causing the bad picture and sound, not the TV. The TV works with the box that's delivering the Picture and Sound, and it could be the box has locked up. Unplug the power cord for a full 5 minutes to ensure your box shuts down it's computer. You always have to unplug any equipment for 5 minutes to do a hard reset.
Next, do this if it doesn't restore normal operation -- Get your TV's model number, and your box's brand and model number too. We need to know what we're working on.
Then check your other equipment connected using the HDMI or COMPONENT input jacks on your TV set. If ALL INPUTS are misbehaving, then we know the TV is where we look, and if only ONE out of ALL inputs is misbehaving, then we look at the box connected to that specific input.
This is a troubleshooting process we call "Divide and Conquer" -- it isolates the symptom you are having.