Data Retrieve

You restore the device from a backup, as I indicated in my original response. The first step after the device will not turn on is this, If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

If it starts, go from there. If it does not start, then you need to contact Apple Support.

What did you do when you couldn’t get the device to open? Did you restore it? If so, the only way to get your data back is to restore the iPhone to your last backup.