Sounds very strange.
Try to clean up your Calibre library before you try to add the new book.
Can you open and read other books in the library? Can you rename books, and still open them? Do you have the library on some external or networked volume? Perhaps you don't have read/write access?
Run library maintenence and check both the database and all the files in the library. Fix any problems.
Verify that you can open the book already in the library.
Verify that you can open the book you intend to add, perhaps with word pad.
Try again to add the book.