This blog post talks about how you can fix an ASUS ZenPad 3S 10 when it wont turn on.
Fixing ASUS ZenPad 3S 10
We managed to fix this issue by:
- Unplugging the tablet from the charger
- Holding the power button for 60 seconds
- Waiting a few seconds
- Then holding the power button for a couple seconds until it comes online
The final button press then turned on the tablet.
From searching around it looks like sometimes it will crash and require a full reboot. Holding the power button for 60 seconds triggers a hardware reset which then allows it to be turned on normally.