Usually that happens with bought books; if you download a book on one computer registered with one Adobe ID,
and then later try to download again to a computer registered with a different Adobe ID.
If you get a new computer, you must register ADE (Digital Editions) with the same Adobe ID as you used before or you won't be able to read the old books.
Is this a bought book or a library book?
Overdrive books are usually library books with a limited loan period, so they don't usually hit this problem.