Please help! I need help with my TV.?

Ok , so i was watching some tv today and i turned it off because i was leaving the room. When i turned it off it was fine. About an hour later the screen is black but i can still hear the sound. I need to fix this fast, fast as in today. Can someone help me and tell me whats wrong please. This tv is about 4 years old. Its a Hitachi Plasma.