Forced to buy a new air ticket - refund needed.

I booked a flight from Riga to Hamburg on 9 May 2015. When I tried to board the flight, I was refused entry because AirBerlin didn't open the ticket. Countless calls later (to both their German and UK branches) I was told I needed to purchase ANOTHER flight. Thankfully this flight was available for me to purchase, however AirBerlin has charged me for the first flight and have not refunded me the money. This is theft. I have paid 134 euros for a service which was refused to me by its supplier. I've already made contact with AirBerlin but with no response. Please correct this.