We have LCD 550 Samsung Model# LN32A550P3F bought it 5/5/2008 has worked great until last night it turns on led backlite comes on the melody plays red lite flashes and screen is black we brought another tv in to make sure dish plugs etc it worked fine on 550 series just black screen and no sound

Hoping it is a menu item that somehow got changed, on page 85 of the manual there is a reset to factory settings. It says to hold down the exit button "for a while" and factory presets will be loaded.
Largest user manual I've ever seen for a TV. Service manuals yes, but user manual?
I hope this works.

This is a fairly common problem among Samsung and it seems to be a Power Supply issue. What has worked for some other people is unplugging it from the power source and let it sit unplugged for at least 24 hours. Plug it back in and see if it works.

Here is another discussion on this issue with Samsung's and the Power Supply:

http://www.dslreports.com/forum/r20889230-Samsung-LCD-TV-No-Picture-but-has-Sound

Don't waste any time on this. Being only 4 months old, it should still be under warranty. Request warranty service or exchange. The power supply module has a problem or the main processing board is telling the power supply to shutdown. In either case, a board change is necessary and should be covered.

Dan