What the error means is that the session table in your database has been damaged. Most of the time this can be fixed reasonably quickly by repairing the table. If you have access to a database management tool such as phpMyAdmin you need to get into it then select your forum database.
Find the sessions table
Select the checkbox next to the sessions table.
Select the repair option from the dropdown options box.
Or if you use cpanel you can repair the MySQL databases, from your Database area
In most cases, this will successfully repair the table.
But remember you need to find what caused it