I always download and keep a local copy of all my eBooks from any source, which would prevent a situation such as this, but this still sucks:
Just got this email from Waterstones UK:
In order to improve the eBooks service that we are able to offer you on Waterstones.com, we are making a small change behind the scenes. This change will result in some lower eBook prices and a wider range of titles for you to choose between, however, some of the eBooks that you have bought in the past will no longer be available for you to download.Our records show that you have some of the affected titles available in your Waterstones Digital Order History, so if you haven’t downloaded these to your computer already, please do so as soon as possible. Please note, when you view your order history only the eBooks with a green ‘Download now’ button (shown below) will be affected by this change, so there is no need to download other eBooks and Audio Downloads from your account immediately.
Read more on the MobleRead Forums.