UPDATE on yesterday: my Sony phone is working again. I found the box and the instructions, which are in Spanish only. There’s a bit near the end which says If Yer Phone is Indisposito, press this tiny contact in the battery area with a ball point pen.
I pressed it but nothing happened. Then I held it down while pressing the power button and bingo, up she came. I was worried it might have wiped all my settings and stored stuff, but it’s OK.
I’ve realised that I can store all the music and images that I used to store on my “iPhone without the phone part”, I’ve forgotten what it’s called. That’s one less thing to take on a trip.