To answer the questions in reverse:
If you saved the image, then closed Paint.NET, then you can't amend. However, if you haven't closed PDN, just press Ctrl+Z to undo any changes, etc.
Also, you most likely had a selection that you were typing outside of, and then pressed Ctrl+D, which deselects it. Of course, I may have misunderstood your problem; I have a feeling that I have.