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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.
@Gordon
Just an update to this if anyone else has the problem - having got it back, it's exactly as I'd said:
Flat battery and firmware update failed halfway through
So just insert a new battery with the button held down, and release the button. The red LED should light as normal for bootloader mode, then you just follow the firmware update instructions again.