How to restore the default toolbox windows in GIMP?

Since you recently installed GIMP and closed the Toolbox window, I would suggest to reset your window preferences because it would also restore the dockable dialogs and the Layers windows which is extremly handy when working with GIMP.

Navigate through Edit menu → Preferences → Window Management → and click the button for Reset Saved Window Positions to Default Values. Then click Ok to close the Preferences dialog and restart GIMP.

I would also suggest to enable the single-window mode so that all these windows would be docked inside the GIMP's main window and ease the workflow when working with it. Enable it through Windows menu → Single-Window Mode.

To open a new Toolbox, just hit Ctrl+b.

rm -r ~/.gimp-2.8

When gimp starts again it regenerates this directory, and you'll get gimp fresh and clean :)