lenovo a7000 power auto turned on after insert battery

using lenovo a7000 android lollipop. once my phone fell down battery and cover came off. but looks fine on the outside nothing break. it turned on automatically after placing back my battery without me pressing the power button. right after booting tried locking screen by power button, no respond for power and also volume button. auto turn on thing happens everytime i remove and replace battery. tried hard reset by pressing power and volume still no respond. only manage to do factory reset in the settings menu, luckily volume button comes back alive. power button dead. please help. thank you.