For fixing the MySQL error:
Admin CP -> MySQL Management -> MySQL Toolbox -> Run following query in the textfield:
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>SQL </td></tr><tr><td id='CODE'>REPAIR TABLE ibf_forums </td></tr></table><span class='postcolor'>
If you need to access your Admin CP manually, this can be done through http://yourboardaddress/admin.php. If for some reason, you are unable to access your Admin CP, (Which may be the case if your members table is the one affected.) you'll have to fix the MySQL error through phpMyAdmin. You can run the query from there. If you don't know the details of where phpMyAdmin is, or if it's even a part of your hosting package, contact your host for these details.
If you do know where phpMyAdmin is, you can select from the left panel the database that IPB is using. Once the main panel comes up, scroll down until you see a textfield that has a label of: "Run SQL query/queries on database yourdatabasename". Run the query there. It should then repair itself.