I can't find my return ticket that I booked for Stansted Express where do I go for help retrieving a copy?
Try logging in
Perhaps you made an account on the site you bought your ticket from. Try logging in and seeing if you can see the ticket there in your profile. If you can, you should be able to do whatever you need to do to access it (print it or transfer it to the app if it's an eTicket, or get the code to print it from a machine if it's a Tickets on Departure ticket).
Contact the site you booked it from
If you know the site you booked it from, contact their customer services. Depending on how far away your journey is though this could be a waste of time as they're not always the fastest things in the world. Tweeting is another alternative which is usually much faster; it's possible they'll ask you to send them details about your email address or perhaps even the card you booked it with.
Ultimately besides these two methods there's little else that can be done other than buying a new ticket. You won't be able to get through the barriers at Liverpool Street without one, and even if you can, travelling without being able to present a valid ticket is prohibited by the Railway Byelaws.