This is a model 3B+, due to some under-power warnings on the screen I was testing the actual voltage between the 5V and ground pins on the header, but managed to touch pin 40 (SCLK) to pin 39 (GND) with the multimeter probe.
This caused the Pi to reset and seemingly be dead - the polyfuse is fine (5v present at test point P7), the red power LED is lit but I don't get anything from the activity LED at all. I've tried the SD card in an identical 3B+ and it boots fine, also tried a fresh SD card with a new install of Raspbian, no help.
Have I just killed to the board, plain and simple, or is there anything else worth trying to resurrect it?
(And if anyone can explain why shorting pin 40 to ground would have such a dire effect, I'm also curious...)