Often atimes, one might find his / her device stuck in fastboot mode. Whether you purposefully booted into this mode or not, you should be able to exit using any of the methods below
Simply Press and hold the power button on your device till the device restarts
Reboot using Hardware Buttons on Nokia Phones
Simply Press and hold Vol up + Vol down + power button on your device till the device restarts. Once the Nokia logo shows up, first release the power button then subsequently both vol buttons
Power Off by Removing Battery
Simply take out / reinsert the battery of the device, then power it back on
Power off using Research download tool for Unisoc / Spreadtrum android phones