Planted a Victoria plum tree some time ago in a black plastic recycle box.
Its been growing really well. when i came to move it (i put it in the
box because i knew i would move it) i found a long tap root (about a
finger thick) had escaped from the box into the soil below.

I cannot get this root out of the ground without breaking it.

Would this result in serious damage to the little plum tree or would it
recover from this? Thanks.