Reviving my phone after a small part of it (eg the processor) overheated
I managed to turn my phone on by myself. Hopefully it will continue working.
I warmed up 2 plates in the oven to 80 Degrees C, took them out, and put a third (cool) plate with the phone on top of them. Once they had cooled enough to hold my finger on them, I stacked them into a warm plate-phone-warm plate sandwich, with the top plate upsides down. This warmed the phone up quite a lot, and was meant to allow any moisture to escape. Maybe it also affected the battery. In any case it turned on after 30 minutes of warming.
I would have put my phone on the radiator, but the weather is too warm to turn the radiator on right now.