We think that the advice that was given by Monty, is not a good idea, it can cause another error and it will be difficult to fix it.
The idea is the same, but the table should be cleared. You should clear all Flat Tables; there can be several of them in phpMyAdmin:
SET FOREIGN_KEY_CHECKS=0; TRUNCATE TABLE catalog_product_flat_1; TRUNCATE TABLE catalog_product_flat_2; ... SET FOREIGN_KEY_CHECKS=0;
But before this action you should write catalog_product_flat in the filter in the left in phpMyAdmin and it will show you all the tables.