Which joomla version? More details are necessary.

2.5.11

I suppose it isn't a new Joomla instance. Please generate and publish data from Joomla Forum Post Assistant (FPA) and describe your problem with maximum details (eg. Provide steps to reproduce the problem) on Joomla International Forum - we will try to help you. You can found more informations about FPA here: http://forum.joomla.org/viewtopic.php?f=621&t=582860&p=2371873#p2371873 . You can try as well to reset Joomla administrator password http://docs.joomla.org/How_you_reset_an_administrator_password%3F
Best regards

After the update to Joomla 2.5.13, only Supers Users can access to the administration backend site.

To provide access again I restored the old code in the following lines in file "libraries/joomla/access/access.php" (new code has been put in comments) in order to take care again of the "$recursive" variable.

public static function getAssetRules($asset, $recursive = false)
{
    ...
    // If the asset identifier is numeric assume it is a primary key, else lookup by name.
    if (is_numeric($asset))
    {
        $query->where('(a.id = ' . (int) $asset . ($recursive ? ' OR a.parent_id=0' : '') . ')');
        //$query->where('(a.id = ' . (int) $asset . ')');
    }
    else
    {
        $query->where('(a.name = ' . $db->quote($asset) . ($recursive ? ' OR a.parent_id=0' : '') . ')');
        //$query->where('(a.name = ' . $db->quote($asset) . ')');
    }
    ...
}

Has it been simply removed by error ?

Closing as a known issue resolved by http://docs.joomla.org/Fixing_the_assets_table