iPhone 6 plus - weird horizontal lines on screen, won't stay in DFU mode for updates, iTunes stuck "waiting for iphone" I need help with fixing an iPhone 6 plus.

The user(my mom) accidentally booted the iPhone into recovery mode(not sure, but she says she's holding both power and home button) and leave the phone on as she didn't know how to exit recovery mode. After exiting recovery mode, it started to reboot, horizontal lines appeared on screen, pixels flashes, and the phone showed no sign of booting up. iTunes said the phone had some problems, and told me to update it. So far so good. But as iTunes was preparing the phone for update, display screwed up again and iTunes was stuck waiting for the phone.

As far as I know, the phone was using ios 9. It's just a guess though, and I do not know the exact sub-version. 

Has anyone experienced such problem? Any help is greatly appreciated. Thanks.
Problem solved- it seems that the battery has been over-drained and a long recharge is needed- I left it charging overnight and it comes to life again after 8 (12 actually, has been charging it before asking this question) hours. This could explain the lines on display- low voltage.