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Be sure the outlet is hot, and that you're connecting the TV straight to it... no power strip. Also be sure the power cord is snug in both receptacles (TV and wall outlet). If your issue persists, this is 1 of 3 things... a faulty power cord, the line fuse, or the power supply board.
If you have some skills and tools, use an ohmmeter to measure
the cord A to XXXXX to B, and C to C... make sure each is zero ohms. A to B, etc. should be infinite ohms. If not, replace the cord. Also be sure it's snug in the receptacles.
If the cord is good, check the fuse.. it's on the power board which will be the first circuit card you run into while tracing the input power cable. You'll need to remove the TV's rear panel first. A good fuse should measure zero ohms.
If the fuse is fine (or if replacing the bad fuse didn't help), replace the power board. You can get a new one from encompassparts.com.
If you'd rather not try this stuff yourself, feel free to post back with your zip code and I'll see if I can find an authorized tech near you, ok?