It depends what is going wrong:
1) You have lost the books, and don't have any copy.
Adobe does not keep a cloud copy of all the books associated with your ID.
It is your responsibility to keep a backup.
You may be able to download them again if you contact the book vendors.
2) You have got the books but they won't open on ADE (Digital Editions).
In this case, make sure your new download of ADE is registered with the same Adobe ID you used before when you first accessed the books.
I tried to re-download the book from my vendor website (kobo). I download it and save the file on my computer. Then I tried to drag the file in ADE and I got this error message. Where should I registered the book? I don't see that?
Thanks for your help!
Then I tried to drag the file in ADE and I got this error message. Sorry, which error message? I have got confused in the thread.
The book was automatically registered to the Adobe ID that you used the first time you accessed the book.
That cannot be changed.
You need to make sure your new download of ADE is registered with the same Adobe ID you used before when you first accessed the books
(using menu/Help/Authorize Computer)
It may be that ADE has got confused registration in which case deregister first (ctrl-shift-D, cmd-shift-D on Mac), then reregister.