What is maintenance mode?
If the micro:bit is connecting as a drive called MAINTENANCE instead of MICROBIT, then you are in the maintenance mode provided for updating the DAPLink interface. The way you access maintenance mode is by plugging in your board to your computer's USB port while holding down the reset button. If your board is in maintainence mode by mistake, this might be how it happened.
If you try and flash a normal hex file to the micro:bit whilst in maintenance mode, it will fail and result in a FAIL.txt file being created with an associated HELP_FAQ.HTM which when opened will direct you here.
How do I exit maintenance mode?
To leave maintenance mode, unplug the board from USB, making sure the Reset button is not pressed, and plug the board back in.
The board should now show up as a drive MICROBIT, indicating it is back in normal operating mode. If this fails to work, please update the micro:bit firmware.