Sometimes you can “loose” your toolbox in Gimp through inadvertent clicking or dragging the toolbox close to the red X in the right top hand corner. This can be easily fixed.
Click the “Windows” menu choice at the top of the GIMP screen, then hover the mouse cursor over “Recently closed Docs”. The fly-out menu shows any toolboxes you recently closed. Just click on one of them and your lost toolbox will be available again.
If you are really totally lost, you can restore GIMP to it’s default (Out of the box) state in six clicks, by clicking on “Edit” – “Preferences” – “Windows Management” – “Reset Saved Windows Positions to Default Values” which brings up a popup window confirming what will happen when you close then restart GIMP. Click OK on the popup box and then OK on the main dialogue box. Below some capture visuals with the “click” numbers.