Mine did that and it was at around 50% battery.
I charged it anyway for around an hour and then it powered on OK (phew). The battery said 90% on poweron, so I know for a fact it wasn't flat when it 'softbricked' (as an hour will only charge the battery halfway or even less).
I was running stamina mode, not sure if its got anything to do with it.
Really hope it doesn't repeat otherwise I have to return it.... or is this software bug....