Try doing a repair on the database via phpMyAdmin ...
If that does not work the table can be rebuilt with:
PHP Code:
DROP TABLE IF EXISTS whos_online;
CREATE TABLE whos_online (
customer_id int(11) default NULL,
full_name varchar(64) NOT NULL default '',
session_id varchar(128) NOT NULL default '',
ip_address varchar(15) NOT NULL default '',
time_entry varchar(14) NOT NULL default '',
time_last_click varchar(14) NOT NULL default '',
last_page_url varchar(255) NOT NULL default '',
host_address text NOT NULL,
user_agent varchar(255) NOT NULL default '',
KEY idx_ip_address_zen (ip_address),
KEY idx_session_id_zen (session_id),
KEY idx_customer_id_zen (customer_id),
KEY idx_time_entry_zen (time_entry),
KEY idx_time_last_click_zen (time_last_click),
KEY idx_last_page_url_zen (last_page_url)
) TYPE=MyISAM;
NOTE: if using database prefixes you would need to alter the table name from:
whos_online
to be:
yourprefix_whos_online
in two places before running that in the SQL box ...