1. TLDR ASUS All-In-One was not booting - just black screen.
Possible Fix - New CMOS CR 2032 battery at horrific location.
2. Background
ASUS All-In-One PC Model: ET2321I
Booting was flaky for several months. Would
halt in BIOS scrolling info, and need function keys
pressed to finish booting.
After 3 weeks of power off it failed to boot.
The screen powered on after power cord was plugged in,
but boot button failed - screen stayed dark after
many permutations of button press:
1. Short press
2. 40 second press
3. Short and long presses with power cord out, then plugged in.
3. Testing power
Ohmmeter detected about 19V DC - good.
Ohmmeter on power button (after case removed) showed 3V DC
before press, and 0V after press - seems OK.
4. Case and motherboard cover removed.
https://www.youtube.com/watch?v=pnF7c7tAN9A
A. Two screws under CDROM. Use paper clip to open
Plastic in CDROM hard to detach - broke small plastic
piece on second detaching.