TV not starting is a common problem. Popular reasons behind TV start up problem are faulty power supply, problems in motherboard, faulty inverter board. There can be other reasons as well. The price of replaceable parts varies largely with brand.
LED or LCD display problem, your first action should be to turn it off and wait for 10-15 seconds before restarting the device. Even though LED and LCD TVs are brighter, more vivid and consume lesser energy and are more stable than their CRT counterparts, they have their own set of problems.
Sound problem in a LED/LCD TV can be due to various reasons like faulty inbuilt speakers, damaged sound card, faulty motherboard etc. There can be other reasons as well.
Plasma TV is more superior and modern than any other type of television out there. You’ve spent a good amount of money for a new plasma TV screen.