Don't worry! Just click on "Rewards History" and a window will pop up with all of your available reward discount codes under the "Status" column so you can copy them again. :) See below!
Step 1: Click on "Rewards History"
Step 2: Your Rewards History will pop up, locate your corresponding promo code
Step 3: Copy the code, and apply it at checkout OR navigate back to "Manage Subscriptions" and apply it to an active subscription by clicking "Edit" and then "apply discount"