Hi,
this can happen if the Library item no longer exists in the database. It is possible to re-create the library item by opening the Mezzmo.db file in an sqlite editor such as SqlLiteSpy and running

INSERT INTO "MGOPlaylist" VALUES(0,'Library',16,'Your collection of music files, video files and photos',-1,0,0,':1:',NULL,NULL,NULL,NULL,0,NULL,0,0,NULL,NU LL,NULL,0,NULL,1,1,0,1,'',NULL,NULL,0);

If the Music, Video or Photos playlist is also missing you can run the following sql:

INSERT INTO "MGOPlaylist" VALUES(3,'Music',16,'Playlist for all your music files and music playlists',0,8,61,':4:',NULL,NULL,NULL,NULL,0,NULL ,0,0,NULL,NULL,NULL,1,NULL,2,1,0,1,'',NULL,NULL,0) ;

INSERT INTO "MGOPlaylist" VALUES(15,'Video',16,'Playlist for all your videos and video playlists',0,10,62,':12:',NULL,NULL,NULL,NULL,0,NU LL,0,0,NULL,NULL,NULL,2,NULL,14,1,0,1,'',NULL,NULL ,0);

INSERT INTO "MGOPlaylist" VALUES(30,'Photos',16,'Playlist for all your photos and photo slideshows',0,9,63,':20:',NULL,NULL,NULL,NULL,0,NU LL,0,0,NULL,NULL,NULL,3,NULL,20,1,0,1,'',NULL,NULL ,0);