iPhone 6 dropped, wont turn on

Hello at the community,

I'm reading your postings and answers with a lot of interest. I like iPhones :-)

I hope someone from the community can help me with my iPhone 6.

The iPhone 6 dropped and the display was broken, it was completely black. I put an orig. Display from another working iPhone 6 on the dropped one, disconnected the batterie before change. The dropped iPhone did after the Display-Change nothing, like after the drop. It won't start (wether with apple logo nor itunes symbol) an no signals are there (no signal when connecting the lightning-Charger, no vibration).

The iPhone won't after the drop start, wether with nor without an display. I can't start the Recovery or DFU-Modus. I've tried the version of holding power- and homebutton longer than 10 sec, but no response.

Now I put another battery into the iphone, but didnt start, too. After about 30 min. connected with orig. cable to wallcharger, no response. After I connected it to my compute, there is still no response, wether recognized at USB-ports nor in iTunes. I tried serveral of computers and orig. cables, renewed drivers, delete folder lockdown etc. My other iPhones are detected without problems.

I got an schematic and can start measurement with a multimeter, that is driven by an 1,5 Volt battery. I can remove EMI-Shields for measuring, that's no problem.

Something that I can't understand or explain:

The working battery I put in the dropped iphone had after disassembling 0 Volt! I think, that the protection circle of the battery is on now. At the inner pins of the batterie I can measure 3,8 Volt.

Dear community, I've written a long text. Thes are all facts an tests I've done about my dropped iPhone 6.

I would be very happy if somebody from the community can help me to get the iPhone working again.

THANKS and a nice evening.

Best greetings MB