Last night the UPS was showing normal operation with 100% battery charge and a ~50W load. This morning the display is completely off, no LEDs are on, the output is off, and it does not restart when the front panel buttons are pressed. Disconnecting and reconnecting the battery and mains input does not help. If the battery is connected the fan does spin though so it's not entirely dead, but nothing else turns on.
This is the second time it's happened, and last time it recovered after being left completely unpowered for some time which suggests there's a thermal cutout issue, but the unit does not feel warm at all. Does anyone have any suggestions as to what the problem could be?