Just to rule out the obvious. Could it be that the button for switching on has been accidentally moved to the "lock" position?

I've only ever experienced the reverse in mountaineering. When the lamp is unintentionally held in and the battery is then empty. Only happens once... 😂

You can't charge the headlight at all:-):-)
No, I'm joking, it works perfectly for me and I've probably had it for 4 years already