To switch your gizmo from Rotation to Translation, click on the Translation icon (circle with 4 arrows pointing out) about 2/3 of the way across the top toolbar. Or you can toggle by using keyboard shortcut [M]. To remember this, I like to think of it as toggling between the two different ways you can MOVE a piece.

Milo, I edited the original post and added a bit about the keyboard shortcut [M], which is probably what your brother smashed.

Ah, thank you, that's very helpful!