How do I solve this MySQL error #126?

I can't update parts of my shopping cart as I get the following error

MySQL Error Occurred

Error Message:
126: Incorrect key file for table '/tmp/#sql_9ebaf_0.MYI'; try to repair it

SQL:
SELECT * FROM CubeCart_options_bot INNER JOIN CubeCart_inventory ON `product` = `productId` ORDER BY `name`, `option_id`, `value_id` ASC

My host has increased the tmp folder size but I'm still experiencing the problem.

I read that error #126 means the index file has crashed - I don't know if this is correct or how I can fix it.

Can anyone help? I need to update my cart but can't do anything until I sort this out.

Thanks for reading