Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
It depends on the TYPE of TV you have, and the Brand name and the Model number, usually found on a sticker on the back of your TV set.
What we usually do to isolate the problem --
1] In a dark or dim room we stare at the TV screen from the moment we press the power button. We look to see if the Pitch Black screen lights up a a subtle gray light level, and watch it to see if it goes black again within 5 seconds or not.
2] If it does stay lit...we press the TV's menu button to see if the menu appears on the TV screen.
Right now with the information you provided, it could be --
Ymain or Xmain (plasma)
Inverters or Balancers (LCD)
High Voltage or Sweep (CRT)
Main board (LCD or Plasma)