Flights tickets

I bought flights tickets from British airways last year and unfortunately my trip was postponed due to personal reasons and I received voucher which I was told I can use them whenever I'm ready to resume my trip and December I decided to resume my trip to CPT but to my surprise when I wanted to buy a plane ticket with the voucher I couldn't find any domestic flights, till I decided to go to the airport to my surprise we got there and the security told us that British airways packed and left so how do I get my money back what they did is really unprofessional and unfair.