Hello there Markpier,
I understand you're curious about the message shown at the bottom of your iPhone, so check out the following information.
With the message you're seeing, this is letting you know that Find My can access the location of the device in the event your iPhone were to be lost even if it's powered off. For additional information on this, check out step 5 of the section labeled "How to turn on Find My for your iPhone, iPad, or iPod touch" of this resource:
Set up Find My on your iPhone, iPad, iPod touch, or Mac
As for your iPhone not powering on, use the steps of the following resource to see if this will allow you to power your iPhone on:
If your iPhone won't turn on or is frozen
If you need further assistance with this, Contact Apple Support to continue working through what you're experiencing.
Have a great day!