Aegean Airlines Denied Boarding Due To Overbooking Despite Check In

I had a serious problem with my Aegean Airlines booking with reference number 74F37C on my flight from Belgium to Athens today. Although I arrived at the airport at 9:00 and completed my check-in on time, I was informed at the airport that the aircraft was full and I was not given a seat, so I was not allowed to board the plane. The staff mentioned a procedure similar to giving up a seat voluntarily, but I was not a voluntary passenger giving up my seat; I was simply denied boarding even though I was on time and properly checked in. This situation caused me serious victimization and disrupted my travel plans. I request a full refund of my ticket for this flight and appropriate compensation for being denied boarding despite having a valid reservation and having completed check-in on time. I ask that my grievance be addressed as soon as possible and that a clear written explanation be provided as to why I was not allowed to board.