ipad air 2 run out of power and can never be charged or turned on

my ipad air 2 was laid aside for many days and it run out of its power finally, now when i plug in my 2.1A power charger ，The following phenomenon appears：

1 the picture of black apple with white background showed up for 0.5 sec

2 the screen then turned dark with backlight (and nothing else)

3 the right hand side of the ipad became more and more hot

4 no button works at all ,including hold the power and home button for 15 sec

5 connect the ipad with computer , no device is detected

6 the official personnel tell me the mainboard is damaged

so why will the mainboard damage itself and what can i do now?thanks~