There is no picture and no sound.

Crazy as it sounds, try unplugging from wall, waiting 1 minute and plug back in. If set up to network or cable, try unplugging router, cable box, etc, same thing 1 minute and then plug back in.
Referred to as a "soft reset".

If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/

Be sure all your input cables are securely attached at both ends, make sure your internal speakers are not turned off.
Reset the TV by unplugging it for 30 seconds to 1 minute.
Also if you are using a cable/satelite box reset it by unplugging it for 30 seconds to 1 minute.
Do you have sound on another input such as DVD player.
if still no audio may have defective I/O-Main PCB

Try to unplug set and a box if used from power for several minutes and then power back to see if a reboot will help or not.

First thing to try is if using a box for signal unplug both that and the set from power for several minutes and then plug back, it is possible the issue will be gone and the set working normal again.

Unplug the set and a box if used--wait for several minutes and plug both back to see if they will reboot.

Can you tell me which Cable box you are using?Which model/region?

The possibility is that the HDCP handshake could not have happend correctly and hence the issue. Try to connect to different HDMI input of TV from Cable box. If possible change the resolution of cable box and check video

Have you tried resetting the cable box by unplugging it for 5 sec. and plugging it back in? (power cable)

I just called Vizio and they said to unplug the TV, then physically hold in the power button on the TV for 2 minutes.