Computer powered on, but screen black

[TRBR]

HP All in One desktop unable to boot/screen black. Was working two days ago without issue, wasn't working the next day. Left the power unplugged overnight, and when re-plugged in the computer started  up and started a very long update. It was at 26%, we walked away and came back to the screen completely black. Waited a bit but still nothing appeared on the screen. We've tried unplugging and replugging. When pressing and holding the power button, it never actually turns off. The light of the power button is constantly on. 

Please help!

[itsmyname]

>  started a very long update. It was at 26%, we walked away and came back to the screen completely black. 

Yes, sometimes, Windows Update can install a "major" update -- usually once every six months.

During such an update, the computer can switch the monitor to "power-saving" mode, e.g., "black" screen.  In that status, touching the SHIFT or ENTER key usually "wakes" the screen, back to full power.

After unplug/replug, what happens when you press the ON/OFF button? Does anything appear on the screen?

If the original HP Warranty is valid, contact HP Support to get it fixed.

Otherwise, take it to any qualified computer technician, for professional trouble-shooting.