This error occurs due to the table jtablesession crashing, and solution is quite simple. Just login to yuor phpMyAdmin panel and run the following mysql query to recreate the jtablesession table. BTW, this is fairly common error and regularly faced by the Joomla websites.
Just import the following table using any MySQL client:
DROP table IF EXISTS jos_session;
CREATE TABLE `jos_session` (
`username` varchar(150) default '',
`time` varchar(14) default '',
`session_id` varchar(200) NOT NULL default '0',
`guest` tinyint(4) default '1',
`userid` int(11) default '0',
`usertype` varchar(50) default '',
`gid` tinyint(3) unsigned NOT NULL default '0',
`client_id` tinyint(3) unsigned NOT NULL default '0',
`data` longtext,
PRIMARY KEY (`session_id`(64)),
KEY `whosonline` (`guest`,`usertype`),
KEY `userid` (`userid`),
KEY `time` (`time`)
) TYPE=MyISAM CHARACTER SET `utf8`;
Hope that helps.
