Device not powering on

So, I’m looking for any answers to shed some light on this…

I have an iPhone 6s and back in April upon charging it with a power bank… with a battery case whilst I was sleeping (I know, dumb idea. The power bank does shut off after the device is fully charged).

So I wake up to my alarm as normal and I was replying to a message also at the time. After I saw it was fully charged, I immediately unplugged it from the power bank, and to my surprise, the screen just blacked out completely. The phone became unresponsive to buttons pressed, chargers plugged in etc… not a sign of life.

The phone hasn’t been touched until a few weeks ago. I was 100% certain that the battery had just fried itself, so I got myself a replacement battery and tested it, nothing. But… having the charger plugged in, I can feel the electricity run through the phone on the back of the phone, like static, but nothing on the display. I’ve tried another display from a parts phone and still nothing. I’m running out of options here and there is some valuable data that I’m not 100% sure was backed up to iCloud.

Any advice / help would be appreciated.