Backlight works, but no image

Hi.

While I repair this MBA 2009, checking devices, suddenly funny beep sound beeping, then display gone black.

That moment, I thought just gone black screen, so I rebooted but display not shows screen. Backlight works fine and boot up OK. Even fn+f1 or fn+f2 could dimming the backlight. Only not image.

Also I tried external display, but that's not work either. No signal.

In this case, what should I check and what could be wrong? By the way, there's no sign of either visual-physical damage or funny smell.