No image on built in display
This Zenbook came with Windows 7 Pro installed. It was upgraded to Windows 10 about 8 months ago, and worked fine for the first 6 months - no issues at all. For the last 6 weeks it was used as a desktop computer with external monitor via the micro HDMI port and the laptop screen switched off using Function+F8 key. Now that I want to use it without the external monitor, I can't get the ultrabook screen to display anything (nothing even when starting up). There's a dim glow from the backlight but no image. Apart from that the thing still works perfectly with an external monitor.
I updated the BIOS to version 219 and the Intel HD 4000 graphics adapter to the latest version from Asus website. I also tried disconnecting the battery to drain the power from the CMOS chip. There hasn't been any accident or rough treatment and the device is 4 years old, not extensively used.
If a hardware problem, then the suspects would be the display panel or the internal cable. I don't know how to check either one.
My guess is that it's a software problem related to using the external monitor or possibly windows updates.