On my Mi4c I experienced the same thing few times during the last year: I eventually found out that the battery was simply completely empty. It can be a faulty usb cable, or maybe the usb port got loosened, or a faulty charger, or something in the system got stuck... whatever the reason, the phone shows it's charging but actually it doesn't, the phone does not shut down immediately at 0% because it thinks it is connected to a charger, and the result is a completely dried up battery. This could be dangerous, as batteries are not supposed to discharge so much.
Your phone somehow reacts when you connect it to the charger, so your usb port should be ok. The usb cable could hopefully be the bad guy here (they wear down surprisingly easily).
Try this.
First thing, be sure the phone is not hot.
Disconnect the phone and try to force-reboot it by pressing an holding the power button for more than 5 seconds. The phone should not turn on, but this should trigger/reset something and make the phone wake up from its coma situation.
Carefully connect the phone to the charger, possibly using a different usb cable (I know usb-c cables are not so common yet, but everyone should really get a spare one) or even a different charger (a 1amp charger may be slower compared to a quickcharge one, but it doesn't stress the battery).
The led will blink red, meaning the battery has not enough juice to turn on the display.
Don't touch any button and leave the phone connected to the charger, the display will eventually turn on by itself showing the Mi logo after several minutes. If in 10 minutes the display still didn't turn on, try to force-reboot it again (this time
The display will then show a battery image and the current charging percentage (it's called "offline charging", as the phone is actually still turned off).
Let the phone fully charge without turning it on.
When the charge will be completed, disconnect the phone from the charger and turn it on.
I know it may sound stupid or obvious. I was really frustrated when my phone was refusing to turn on: I tried and tried but that damn led was always blinking. I let it rest for a while, tried this stupid-proof metod and voilà the phone resurrected.
Hope it might help you.
Inviato dal mio Mi-4c utilizzando Tapatalk