The only time I've seen this happen was with a public library ebook. I was able to read it beyond the due date, but at a certain point, either I turned off my PRS650 or connected it to my laptop and the DRM message came up. The legal options in that situation are:
1) Wait for it to be available to borrow again
2) Buy the ebook
Or was this a book you purchased? If so, was it not possible to just delete it and reupload it to the Sony? If that doesn't work, you would have to contact the vendor who sold it to you.