As mailed to all users involved:

It’s been more then six months since you last logged in to your SiteBar account. As part of our policy, all accounts not used for over half a year will be removed (terms and conditions).

To prevent removal of your account and loss of the stored bookmarks, all you have to do is log in at http://sitebar.beket.nl (password retrieval tool available) in the coming days. You can also export your bookmarks to use them in some other place and remove the account yourself if you no longer wish to use this service.