What has happened is a database crash ...

First, try to do a repair on the sessions table via phpMySQL ...

Note, you may have a prefix on your table name ...

Once you have done the repair, be sure to do a good backup of your database ...

Also, contact your hosting site and see if they can tell you why there was a database crash and make sure this is not a regular event ...

It does happen on rare occaision ... but you do not want to see this happen often ...