My tv only turns on by button on back of tv, but sound and no picture

Graeme Kerridge  while the screen is black and you do have sound, shine a flashlight at an angle against the screen. See if you can make out some shapes, silhouettes etc. if so it is your backlight that is having issues. If not then it is possible that this is a main board issue. Not turning on other than the button could be an IR board issue or a main board issue as well. Let us know if both of these errors occured at the same time. we also need to know the exact model for your TV.