A frantic Google search last night failed to shed any light on what happened to my mobile phone last night, so I'm posting this blog for anyone else who suffers the same thing.
It began with my mobile chirping at me with an unfamiliar sound; not a call or an event or a text. I pressed it to light up the face and got a brief glimpse of a message telling me that F-Secure was active. Then the phone switched off and wouldn't come back on.
Disaster. Had the phone security program decided that the phone had been stolen and so locked it down?
The phone had spent all day connected to my PC, so it should have had a full battery. However, on the off chance I disconnected, then reconnected it. The battery charging indicator came on, though nothing else was happening. After a few minutes I tried to power the phone on, and it came back to life.
I don't know how or why the micro-USB connection decided to blow instead of suck, but that's what must have happened. The battery got empty, the phone tried to warn me, and the message about F-Secure might have been related or just a complete coincidence.
One last thing worth remembering: even with the phone in this state, I could still access the memory card via the PC.