iPad Mini completely Dead after changing Touchscreen?

Hello,

Today I tried to change the Touchscreen of a customer. The iPad wasnt used for 1 Year, so i quickly check if the Batterysign is there if i put the lightning cable into the iPad Mini. The Batterysign displayed on the Lcd and i drag out the cable again and started the Repair. After Taking off the old Touchscreen i put the new one into place. I put the cable back into the iPad just to check if everything is fine, but there was nothing on the Lcd no Batterysign just black. I removed the Batteryconnenctor and put it back into the connector but nothing happpened. I tried a different LCD and put it into my pc but also itunes cant recognize it. The Battery has 2.7 Volt. And if i Put the lightning Cable into the iPad without the Battery connected there is also nothing happening LCD is still Black. If I measure the Batteryconnector on the Logicboard there is only 0.13 V: when the lightning cable is connected to the iPad. But if i measure the LCD connector the both opposite pins on the right the multimeter shows 3.2 Volt; again the Lightning cable is connected and the battery discconected. If i connect the Battery and measure on the LCD FPC it shows exact 2,7 Volt, like the batterypower.

Im sorry for my english it isnt the best, because im from Germany. I hope anybody can help me.

Or is the Board completely dead?

Thank You!