My 840 v9.29 had 8% of battery so I plugged it to charger. It was in sleep mode. After this screen was enabled and Garmin logo showed. After a few hours display is still on with the same image and 840 do not react for anything. It is dead.
I had the same issue! My new Garmin 840 was attached to the Garmin Charge Power Pack and when I came back to it a few days later, the 840 looked like it was dead. Not responsive with any buttons.
I held the power button for 10+ seconds (I counted beyond 10) and it came back to life. Very helpful. Thank you!