iPad - Battery symbol on centre of screen
The iPad battery has drained completely. Plug in the charger and wait for a few minutes for the iPad to turn on automatically. 

If the iPad doesn’t turn on automatically, press and hold the power button after charging for 15 minutes to power it on.

To check if the battery is actually getting charged, plug in the charger, and press and hold the power button, the indicator on screen will show the device getting charged.