The problem is a corrupt database on the device. The reason is almost definitely the PC crash while the device was connected and you were transferring files with calibre. It can happen at other times, such as not safely ejecting the ereader.
There isn't much you can do to fix this except, as Jon has said, sign out of the device. This will removed the database, take you through the basic setup and then reload the books that are on the device. Unfortunately, you will lose the reading status of the sideloaded books and any change for downloaded books since the last sync.
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