It's like a halfway between the android bootloader mode and recovery mode, generally to fix it, you would hold the power button down for 10 seconds to force the phone to turn off and then hold the power button to restart the phone
I had this today. Message said to hold in power button for 10s and when i did, it switched off and i switched it back on normally. All seems ok although as it happened when i put it on charge last night, there was no power in the battery.
The Volume down for 10 second and Power Button does not work but try to use the Volume up button for 10 seconds and then the power Button which works. Thanks to Casual Gamer for the hint. From Ismaeel Soliu