Multi Vendor - After update to WooCommerce 2.6 all vendor data is missing
This problem is due to the fact that WooCommerce has deprecated the woocommerce_termmeta table in favor of wp_termmeta. The migration procedure is offered by WooCommerce same, just click on the RUN button UPDATER found in the banner at the top in the administration of WordPress, see screenshot.
At the end of the procedure just press F5 to reload the page and everything will go back to work.