The server is down for the time being due to an error caused with a restart - Unfortunately all inventories wiped (enderchests included) and we're updating to a new plugin to prevent it happening again in the future. The server will be up when the new plugin is sorted, and so we'll keep you updated.
There seems to be no way to give you all items back in any easy fashion, but we do have logs of inventories from yesterday (at least in some cases) and so we'll be using those lists to return those items to you manually. Any books come up as logs, and so those can be sent to you via forum conversation. This will take time, and we apologise, but are working hard on fixing the issues.
Claiming some posts too - Will update soon with more information.
A command to restore inventories from the last recorded time will be used on each player, please PM a member of staff so we can get around to you. This will vary, ideally it will be recent as some cases are from yesterday, but it seems to update when worlds are changed (pocket, for example) and so that's when inventories are likely to be restored from. We'll be using the same plugin that we've used for years now until the upcoming 1.13 release, so while we don't expect this error again it may be more ideal to store any particularly valuable items in chests separately. (We also suggest moving things to storage in preparation for the upcoming release. Tiberione has more info on that with his threads.)