Picture Zbrush as a painting program like MSPaint or Photoshop. You select a tool from the tool palette - be it a paintbrush or a 3d model - and draw it onto an image document in order to manipulate the pixels on that document.
But before actually commiting the result to the document, the very last drawing you did will stay in a waiting state as zbrush will give you a chance to adjust it (by moving, rotating, or scaling it, or the special fourth option of edit mode for 3d tools which is what most people use it for). Only the most recent drawing can stay in the waiting state though, so as soon as you make a new drawing on the document (even an empty click of the mouse), then the previous drawing is converted to pixels.
So if you can’t edit what is on your screen, then it has already been bumped out of the waiting state and converted from polygons to image pixels. To resume sculpting:
- Clear pixels from the document using Ctrl+N
- Select your tool from the tool palette (it should be saved with the project, if you can't see the thumbnail of it you may have to use the slider).
- Redraw the tool back onto the document
- Enter Edit Mode while this tool is still in the waiting state.
Note: There is a small possibility that a project has not saved a Tool even though its supposed to. If you don’t need anything specific to the larger Project file format then you might be better off just saving the Tool itself (.ztl file). Don’t save the Document as that is the 2d canvas you’re technically painting on; it contains no 3d data and is not something you’ll often need while sculpting.