Here are a few more troubleshooting steps you can try when your TV picture is just snow. I see that you have already rebooted your TV, but did you turn your set-top box off and on, too?
1. Does the picture show when you play a DVD?
2. Check all of the connections between the TV, outlet and set-top box and make sure everything is secured.
3. Is your TV set to the correct channel (typically 3, but in some areas 2 or 4)?
4. Press any key on the remote to see if the correct mode lights up. (Ex: if you have cable, the cable button should light up). If it doesn't, press the correct mode's button to regain control and change channels.
5. Call your service provider and make sure it's not an overall service outage.
6. Do you have sound but no picture? If so, it could mean a backlight failure.
For more suggestions, see TV black screen or snowy screen
for troubleshooting and solutions. I hope this helps!