Ah looks like your database file has gotten corrupted somehow. There are ways you can try to restore it. Install the sqlite command line tool from
http://www.sqlite.org/sqlite-3_6_18.zip
then run the command
sqlite3 metadata.db > dump.sql
(metadata.db is the sqlite calibre database it will be in the calibre library folder)
This command (if it succeeds) will create a file called dump.sql
Run
ren metadata.db metadata.db.orig && sqlite3 metadata.db < dump.sql