Thought I had lost all my ebooks when I opened Adobe Digital Editions to find a blank library. For future reference for anyone else who experiences the same, follow this advice:
Next to 'Library' click on the downwards pointing arrow.
In the drop down menu click on 'Add item to library' which will open up your 'Documents' folder.
Open the folder called 'My Digital Editions'
Select the ebooks you want to restore to your Digital Editions library and click 'open' which will put the ebook(s) back into your library.