Thanks!

Hmm. That one just says unplug the battery and plug it back in again. That's what I did.

The second one is too esoteric for me. I have no idea what it is on about. Never used adb, not would I know where to start. If I really had to go down that route, I would rather buy a new phone.

The last link did not any useful suggestion. "Use a different charger". "Tried it, did not help." Well, duh.