From Books on Boards webpage:

Please sign in on the login page to download books you have purchased. BooksOnBoard has temporarily stopped selling books. We are re-structuring to compete more effectively against Goliaths who have entered our marketplace since we first launched in 2006, i.e., Amazon, Apple, Google, Sony and Barnes & Noble.
We expect to be back with our new business model soon. Meanwhile, customers can continue to download their prior purchases as long as the publisher and their distributors continue to make them available – same as always and out of our control. As in the past, it’s always wise to download your eBooks as soon as possible after purchase.
Thank you for your support of our store over the last seven years during which we have pioneered many innovations with our customers, including the first eBooks for sale on the iPhone. We look forward to serving you again with an exciting new approach soon.

The BooksOnBoard team

I am not surprised, honestly, since they have been very generous with the discounts for the last 2-3 months.  Not to mention publishers have complained about not getting paid, etc.

Will they be back?  I am not sure, honestly. Sure, they plan to be back, but today’s e-book market is so competitive that it might be difficult for them to win back their old customer base.