If you have an On Hold order you're ready to put back into Outstanding status, you'll need to select "Move to outstanding" to add it back to the fulfillment queue. This is completed under the On Hold orders tab of your Orders & Fulfillment page.

If your order cannot be fulfilled by Pietra for any reason, for example if an item in the order is out of stock, the Pietra team will place the order back On hold for your review. After you complete the required update, you may "Move to outstanding" again place the order back into the fulfillment queue.