After being moribund for several years, Amazon is finally going to close Mobipocket.com at the end of October.The annoucnement on the Mobipocket home page reads:
"The Mobipocket website & its services will be permanently shut down on Oct 31st, 2016. Prior to that date, please ensure:
You have downloaded copies of all the ebook content you've purchased through Mobipocket. Please re-download any ebooks you need prior to October 31st.
You have activated all your Mobipocket CDROMs. You can activate CDROMs by clicking here."
I think I only ever bought one book from the actual Mobipocket.com web site, and trying to download that now just gives a server error.
But if you had used Mobipocket in the past, and you don't have current backups, it might be worth seeing if anything is recoverable from your account before the end of October.