Digital Editions can't open books authorised under old account

I recently upgraded to Digital Editions 3; this was after upgrading my hardware which required me to re-download Digital Editions, during which I lost my entire library (no big problem I thought, as I had all the original documents and downloads).

I had purchased an e-book through Google and wanted to transfer it to my e-reader, but was unable to do so. I was advised to re-authorise my computer, which I did, but now none of the books I had will open. I get an error message "This document is licenced for a different user account". Adobe "support" is no help at all. Has anyone else encountered this problem?