If your micro:bit is showing up as "MAINTENANCE" in your File Explorer window, rather than "MICROBIT," then your micro:bit is in maintenance mode. This can happen when the reset button on the micro:bit is pressed while plugging in to your computer's USB port (see image below).
In order to leave maintenance mode and be able to download your code follow these two easy steps:
Unplug the micro:bit from the USB cable, making sure the reset button is not pressed, and plug the micro:bit back in. The board should now show up as a drive titled “MICROBIT,” indicating it is back in normal operating mode.
If Step 1 wasn’t successful, you’ll need to download new firmware onto the micro:bit. Plug in your troubled micro:bit and click the purple “Download the latest micro:bit firmware” button about halfway down this webpage.