Normally no refund is allowed by the airline if you do not cancel your flights in advance of the outbound flight.
If you do not cancel before departure and you do not turn up for the flight, this is considered by the airline as a "No show". In these circumstances it is unlikely that the airline will authorise a refund and the airline may also cancel your return flight automatically.
On the day of departure, if you know that you will miss the flight, speak to the airline as soon as possible, to see what alternative arrangements can be made and to ask them to hold your return flight.