After restoring windows to an earlier date settings, my ADE disappeared, after reloading and trying to re-authorize my computer I get the message: "The vendor account you entered is not associated with the item you are trying to open." and the vendor account line is greyed out
Sometimes ADE gets its registration/activation confused and in a semi-authorized state. This is almost certainly what happened here.
Uninstalling and reinstalling does not help.
Unfortunately, it often then gives misleading error messages about what is wrong.
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.
In extreme cases on the mac, the following extra step has helped some people. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash. If you are using 10.7, see Access hidden user library files | Mac OS 10.7 Lion. http://forums.adobe.com/thread/1265248?tstart=0