Can I sue the airline if they wouldn't give me my discount?

I was in Argentina and there was a strike at the airport 3 days ago, I was 3 months pregnant at the time. The number of hours spent there had an impact on me. Upon arrival I went to the hospital and had a miscarriage there. The airlines issued me a 30% discount on a future flight and sent me a letter confirming this. I contacted the airline end of 2011 to get a price but they couldn’t give me information unless I sent them the original letter. Therefore, I did but kept a copy for my records. I sent the letter certified. The promised discount had no expiration date. Unfortunately they no longer offer a flight from New York and having to fly to Florida first was going to end up costing me more than if I flew with another airline. So I let it go but they wouldn’t send me the letter again and said it would be fine for when I wanted to redeem the discount in the future. So I recently called the guy to get the flight with the discount and he denied getting the letter and I couldn’t find my certified delivery slip and he refused to follow through with the discount and wouldn’t let me speak with anyone else. He even interjected my call when I called the airline directly. Well I found my slip now.