Do you have your old computer still. If so, go to digital editions on it, and in the Library view do menu/Library/Authroize Computer.
That will tell you the email it was registered with.
There is a long-winded way (below) to find the internal AdobeID a book belongs to,
I don't know if having got that internal ID whether Adobe service are able to tell you what the external ID (email) assocated with the internal ID is.
I have made this comment before but nobody has responded whether it helped or not .....
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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
<user>urn:uuid:ff2ddc22-eca0-46c6-a84d-xxxxxxxxxxxx</user>
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.