iPad Mini 2 only powers up on charge but bootloops and then gets stuck

My iPad is on iOS 10.3.3. I cracked he screen and the battery etc was disconnected for a week now. Popped the new lcd and digitiser in and tried switching it on but the device did not switch on. I then plugged it into a wall adapter and saw the Apple logo (finally). But then the device started bootlooping on the Apple logo. I hard reset the device and eventually I was led to just a blank screen. I left it on charge over night with no luck. Whenever I disconnect the charger the device switches off and does not switch on again.