DNX fastboot mode is a special mode that allows your device to communicate with a computer via a USB connection, it's usually used for flashing firmware or troubleshooting. It sounds like your device might have encountered a problem that caused it to boot into this mode. To exit DNX fastboot mode, try unplugging the USB cable and then pressing and holding the power button for about 10 seconds to force a reboot. If that doesn't work, you can try connecting your device to a computer and using the device manufacturer's software to flash the firmware or restore it to its default settings, this should help your device boot normally again.