I was surprised and shocked when Thor's Hammer was broken in to pieces by Hella. But how??? Isn't the hammer is invincible?
To know that first get some background on who Hella is and how powerful she is. She is the goddesses of death and rules the hell.
As per the one of the movie scene where Hella was the one who was using the Thor's Hammer before. Also she has the power to control the Astral artifacts.
Though she was not worthy she was just using her power to break the Hammer.