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Failback default mySQL data if Guestbook havn't create this.

1.) Open your phpMyAdmin webfrontend or an other tool.
2.) Select your wordpress database.
3.) Copy the mySQL statement and put this in the SQL windows.

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Be sure the Guestbook name have the correct prefix.

mySQL 4.1 and above:

CREATE TABLE wp_Guestbook (id mediumint(9) NOT NULL AUTO_INCREMENT,
name varchar(50) DEFAULT '' NOT NULL,
email varchar(50) DEFAULT '' NOT NULL,
gravatar varchar(32) DEFAULT '' NOT NULL,
url varchar(50) DEFAULT '' NOT NULL,
date int(10) NOT NULL,
ip varchar(15) DEFAULT '' NOT NULL,
message longtext NOT NULL,
guestbook int(2) DEFAULT '0' NOT NULL,
spam int(1) DEFAULT '0' NOT NULL,
additional varchar(50) NOT NULL,
flag int(2) NOT NULL,
UNIQUE KEY id (id)
)DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

mySQL 4.0 and lower:

CREATE TABLE wp_Guestbook (id mediumint(9) NOT NULL AUTO_INCREMENT,
name varchar(50) DEFAULT '' NOT NULL,
email varchar(50) DEFAULT '' NOT NULL,
gravatar varchar(32) DEFAULT '' NOT NULL,
url varchar(50) DEFAULT '' NOT NULL,
date int(10) NOT NULL,
ip varchar(15) DEFAULT '' NOT NULL,
message longtext NOT NULL,
guestbook int(2) DEFAULT '0' NOT NULL,
spam int(1) DEFAULT '0' NOT NULL,
additional varchar(50) NOT NULL,
flag int(2) NOT NULL,
UNIQUE KEY id (id))