My iPhone 6SE froze in middle of a phone conversation and has a black screen

There is no model 6SE, so it isn’t clear what model phone you have. I’ll provide advice for the iPhone 6, 6S, and SE:

iPhone 6 and 6S:

Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

iPhone SE:

• Press and quickly release VOLUME UP

     then

• Press and quickly release VOLUME DOWN

     then

• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky.

Thanks for the photo of that memorial. My wife is descended from immigrants who came from Poland around 100 years ago, so same part of the world. And glad I could help.

We have wanted to visit Prague some day, and may still make it.