iPhone 17 Pro Max does not turn on after trying to update to iOS 26.3.

I was showing my dad how to sync his music to his new iPhone 17 Pro Max on a Windows 11 computer. On the Apple-Device app, it showed that an update to 26.3 is available. I told him to update the phone that was connected via the original Apple USB-C cable. The phone got stuck on the Apple logo screen with the loading bar. After some hours, nothing changed. I used the key combo to restart the phone. I tried several times to start the phone, but it did not work. I downloaded the iOS 26.2.1 file but also failed. Now the iPhone does not turn on. Does anybody have the same issue?