Flight Cancellation

I had a flight from Trabzon to Istanbul and from Istanbul to Budapest with Turkish Airlines. When I came to Trabzon Airport they told me and my colleague that the flight was cancelled from Trabzon to Istanbul. They offered us a different flight but when we were late for the next flight from Istanbul to Budapest they did not change this other flight. They made us buy our ticket second time (even return tickets!). So we spent 4 times more extra money on the tickets and were late on the conference in Hungary. For 2-3 months I am trying to contact them with a complaint, but nobody is answering it.