you bought a brand new laptop but when you connect it via HDMI to an external monitor it doesn't work. The external monitor switches to HDMI mode so it recognises the fact something has been plugged in, but the screen stays black. The laptop acts like nothing's happened.

You try a different laptop, with the same HDMI cable and the same external monitor. The external monitor immediately displays the laptop's screen correctly.

The external monitor has a native resolution of 1920x1080 @ 60 Hz.
Your laptop has a native resolution of 1920x1080 @ 60Hz.

You try connecting the laptop to the external monitor by their respective VGA sockets. This works. For a laugh, you try re-inserting the HDMI cable whilst the VGA cable is in. The laptop makes the 'new hardware' sound and the external monitor switches to HDMI, correctly displaying the laptop's screen as you intended from the beginning. You unplug the VGA cable. HDMI still working correctly. You unplug and re-insert the HDMI cable to see if the problem is rectified but you're back at the beginning. Black HDMI screen again.

You contact Acer and describe the problem. They say it sounds like a hardware issue and ask you to send it to them for repair.

You get your laptop back two weeks later. There's a sheet of paper in the box with your description of the problem and then the procedures they undertook to fix it. It says they replaced the mainboard and updated the bios and it passed all tests.

You connect the laptop to your external monitor via HDMI and it doesn't work.