If you Joomla root site show this error :

mossession::store failed
DB function failed with error number 1034
Incorrect key file for table 'jos_session'; try to repair it SQL=INSERT INTO jos_session ( `session_id`,`time`,`username`,`gid`,`guest` ) VALUES ( '3fb0665c11671cb724b8fd4f37439d6f','1187102382','','0','1' )

The message show error in "jos_session" table and this table located at your joomla database at phpMyAdmin.

You can fix your "jos_session" table by running some Mysql code. Here is the step :

Open your Cpanel, at the databases sub menu, click on phpMyAdmin. On the phpMyAdmin interface, select the Joomla database that the problem occurs. Example, if the error of your main site comes from Joom1 database, select joom1.

On default display when you choose your database, it will display the "structure" tab content, then open the SQL tab like figure below:

On the Run SQL query/queries blank form, copy paste this code:

DROP TABLE IF EXISTS `jos_session`;
CREATE TABLE IF NOT EXISTS `jos_session` (
  `username` varchar(50) 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',
  PRIMARY KEY  (`session_id`),
  KEY `whosonline` (`guest`,`usertype`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

Then, click GO button to execute it

Click OK if you really want to execute this command.

If the code work successfully, this figure below will show the message "Your SQL query has been executed succesfully.