Turkish Airlines Return Flight Charge of €500 Per Person

I purchased a round-trip ticket to Bosnia from Turkish Airlines (THY). I missed the outbound flight and bought another ticket from a different airline. We did not cancel the THY ticket, so we paid for it but couldn't board the plane. Anyway, we traveled to Bosnia, and when returning, we checked in online for the THY return flight and went to the airport. Just as we were about to check in our luggage, we were told that unless we paid €500 per person, we couldn't board the plane. We explained that we had a ticket and had checked in, and there was no cancellation of the previous ticket. Yet, they did not let us board. I will file a lawsuit as soon as possible. We had to stay an extra day and spent an additional €600 to buy tickets from another airline. What happened to the money we paid for the flight? It was not refunded. What about the flights? They continued with empty seats. Was there a ticket cancellation? No. We will see you in court.