British Airways (day light *******)

I booked a flight in February to Durban. Due to unforseen circumstances I had to cancel the flight at the last minute, I was given a voucher which I could use on an alternate flight consequently all local flights were canceled, making it impossible to use the voucher or even try and sell it to someone. I had and still have no intention of traveling internationally therefore the voucher became useless to me. I made contact with the airline and they said there is nothing they can do.....the voucher expired at the end of September and all my money went down the drain.... Was it my fault that all local flights were stopped or theirs?