Sound but no picture....
Should ask yourself, does it PRIME? Meaning, That's where the Plasma panel lights up a little for about 10 seconds after startup. -- If it doesn't prime, then you need either the SMPS, YZUS, possibly both. OR you might need to check the ZSUZ. Driver boards can be culprits too.
And can you even get to the Menu, Volume, etc. but just picture?
Then you need the signal board.
Unfortunately that's pretty common with Plasmas. Can burn out or burn "in" easily.
Here's a great forum you might like. http://www.austech.info/electronics/20607-lg-plasma-tv-has-no-picture.html
Different Brand but similar problem. This site is excellent for do it yourselfers. As for Samsung Parts, be thankful if you are a DIY-er that the parts are much cheaper if you install yourself. Separate with installation adds up very quickly. Personally, I decided to go with the Rear Projection Samsung several years ago and haven't had any major problems. Heard then that Samsung was having lots of complications of this nature.
Here's the link to Samsung Site:
You can try the tricks on this site:
To be honest though sounds like either bad SMPS and/or bad Y-SUS pcb's.
Wish you good luck.