In case of connection failure or unexpected issue during an update process, it might be helpful to put your MP500 in recovery mode:

To do so:

1) Power off your device.
2) Press and hold the start button for a few seconds until the LED lights orange (metallic housing) or just blinks (plastic housing).
3) Release the start button. The power LED should blink regularly (both housings).

Your MP500 is now in "recovery mode" and you can try to perform a new firmware update from this mode to restore your device.

After that, power off/on again the device to switch back to the standard mode.