Where did the left-hand toolbar go? It doesn't matter which mode I'm in (object or edit), or which tab (sculpting, modeling), the left hand toolbar is about 1/8th the size it should be, and none of the tools I need to manipulate the objects are there. I'm new to Blender, so I have no idea what I've done wrong. I just want to sculpt/smooth out the cushions on my mesh.

Thanks in advance for your help.
You changed to Object or Edit mode as you said, the combobox(button) where you can change to Object or Edit mode also offers the option for Sculpting Mode.

While the option to change to Sculpting mode is only available if an object that can be sculpted on is selected,(no camera, light, empty, etc..) that is to say generally only mesh objects can be sculpted on.

Here a little video showing the change to Sculpt mode:



Happy Blending.