Do you already have bought DRM books associated with the ID?
If not, just forget it and make yourself a new account/new ID.
If so, you do need to get back the old ID in order to read them.
The ID is probably an email address you use or used to use.
See if you can guess it.
If that fails ... you can get the internal ID ... long details below, which will still only help if Adobe are helpful.
A book is associated with the AdobeID in use at the time the book was downloaded (.acsm token file turned into encrypted .epub/.pdf file).
An AdobeID has two forms, internal (something like 'urn:uuid:ff2ddc22-eca0-46c6-a84d-xxxxxxxxxxxx') and external (email address).
The authorization mechanism on the book is associated with the internal ID.
The internal ID for an account never changes; you can change the email address it is currently associated with on the Adobe website.
You can check the Adobe ID ADE is using in internal and external form in ADE2.0 using ctrl-shift-I (Help/Authorization Information).
For ADE1.7.2 you sadly only get the external ID, using Library/Authorize Computer (which acts as a query if already authorized).
You can check the AdobeID in external form is Bluefire.
In either case, if you have changed the external ID associated with the account since authorizing the device,
you may be told the old external ID, or may be told the up to date one.
You can check the AdobeID associated with a book in internal form with the rather roundabout method at the bottom of this post.
I don't know of any way of finding the current external ID given the internal ID of the book;
unless it is an ID you still use in which case the mathing values from ADE2.0 will tell you.
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.