Ammeter stops reading

I'm pretty sure you have a voltmeter an not an ammeter? But regardless, I'd have to take a look at the electrical layout in one of these. It might be the voltmeter is powered directly by the key but it might not be. In other words "through a relay."

Did anything else seem to be dead? That would be a big clue. I doubt the meter is powered by a complete dedicated circuit, it likely is tied to other components. If the owners manual has a list of fuses and what they serve that might give you some direction

If it simply quit charging, the meter should read low, not zero