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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@epw
It's good to know that attaching the cable backwards won't damage it, I had worried about that.
I do have a voltmeter, and don't know why I didn't think of it! Also that has made the problem clear. The cable is giving something like 0.1V when plugged in. That would, indeed, do it. I never see the screen do anything or the heart rate monitor LEDs on the back, but then I wouldn't, with essentially no power coming through.
So, thanks for the suggestion! I'll just wait until my replacement cable comes in, and hopefully everything will work out then.