A perk of the SAMD UF2 bootloader is the ability to load other firmware (like CircuitPython) onto the board without re-programming the bootloader.

Double tap the Reset button, a new volume should appear on your computer - boardNameBOOT (the name of this volume differs between boards).

Drag and drop the CircuitPython board and language-specific UF2 file we downloaded earlier from the desktop to the boardNameBOOT volume. 

The volume should briefly disappear, and reappear as CIRCUITPY.

Congrats, your board is un-bricked and running CircuitPython again!