Few options are available:
Option 1: Search your email inbox for "email@example.com". Have a look too at your Spam/Junk folder. Once found, click on the link inside the description and re-enter your credential if necessary. Make sure you use the same email address or Facebook/Google access as you did for your initial purchase. It should work!
Option 2: Clear your browser cache. You can find how to do it here. Then follow Option 1.
Option 3: visit the event page and click the link "Lost Your Code". Enter your email address and check your inbox. If the email isn't recognised the form will inform you.
Option 4 (last option): click Live Chat and talk to one of our friendly team members. Please have the last 4 digits of the card number ready so our team can find your ticket.