Pegasus Airlines Check-In Chaos

I purchased a ticket on Pegasus Airlines to travel from Sabiha Gokcen to Mus, but despite my best efforts, I was unable to check in 2.5 hours in advance. When I arrived at the airport, they informed me that I needed to go and take care of it, but there were still about two hours before my flight. After I had returned, they would not let me in, saying that I was too late because the plane was about to take off. I'm going to take legal action if I don't get a proper apology and compensation for what happened to me.