The Adobe DRM system does not keep copies of your books, backup is your responsibility.
If you have the .epub/.pdf files somewhere, that is good.
If you have the .acsm files you should be able to download the .epub/.pdf files, once you get properly registered with the old ID again.
If you don't have either the .acsm or .epub files, you may be able to get them back by contacting the original sellers.
The old ID will be an email address you used at the time you signed up (unless you explicitly changed the email associated with the account later).
There is a complicated way to find the internal ID for your Adobe account.
Adobe should be able to access and reset the account given that information, but I am not sure if they can.
Good luck ....
OK. To find the internal AdobeID.
open a sample DRM .epub file as a .zip file.
With some zip programs, you can just open the .epub as an archive (ev, with 7zip),
with others you will have to rename the .epub file to a .zip file first, then open it.
find the file 'rights.xml' within the zip. Probably 'META-INF\rights.xml'
open the rights.xml file
inside you will find a section '<user>' that looks something like
That string is the internal version for your AdobeID.
That stays fixed for the AdobeID, even if you change the associated email (or password),
that internal ID is what is really associated with the book.
(n.b. there is no point in trying to change the urn to a current one you might find and recreating the .epub file; the DRM is cleverer than that).
You can now contact Adobe at Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html
It may be that if you can give them that internal ID they will be able to get you access to your account again.
If they say they can't, it might be worth trying two or three times at intervals;
it seems from what others have said that some of the representatives are very clued up, but others are not.
(I've never used the Live Chat).
If you do go through all that, let us know how you got on.