In general, full service airlines do not allow you to use only the returning flight as a return ticket must be used in sequence. This means, the outgoing flight must be used before the returning flight can be used.
If you need to use only the returning flight, you will need to purchase a new ticket and refund the earlier purchased return ticket, subject to fare conditions.
For low cost carriers, the returning flight might still be able to use as some low cost carriers do not have the ticket condition that a return ticket must be used in sequence. If you have booked your flight ticket with us, please contact our Reservations Team for assistance. Otherwise, please contact the airline directly for more information.