No single answer is given to this question nowadays, and no single answer was given at the time.
Anyone who has been at a public reading or a dramatization of the Passion of Jesus Christ knows just how many separate parts there are. At the liturgy on Good Friday we will hear the Passion narrative according to St John, and of course the other Gospels give their own accounts of the Passion.
Simply to jot down those who in some way came across Jesus from when he was arrested to when he died on Calvary makes a varied and quite long list. The men and women involved understand and react to what was happening in different ways. There were religious people confronted with an event that did not fit easily into the understanding they had of God and of their religion. There were the disciples and the mother of Jesus who had their own griefs to cope with, and would have their faith tested.
There were soldiers and guards of various kinds who -- at least at first -- probably thought it was going to be just another spell on duty; there were Jewish and Roman officials who had to take decisions, be they religious or political; there were other condemned men, there were crowds; and there were assorted individuals. Some came to see that day as like no other, some no doubt stayed puzzled for a long time, others saw nothing beyond one more public execution.
If, somehow, all those who in some way came across Jesus during his Passion and death were asked at the time 'What has happened?' they would have given different answers. Later, some would have changed their answer as to what had happened.
Which brings us to now, to facing the same kind of question. A survey of people's attitudes and responses to what happened on Good Friday will still give us a huge variety of answers. At one end is the belief that nothing was the same after that Friday, not just for Christians but for everyone. While at the other end there is indifference or ignorance. In between, there are many points of view and so a lot of different answers.
So, 'What happened on Good Friday?' The goodness of Jesus's love and sacrifice at Calvary is immense, permanent and efficacious. It is inseparably connected to his life on earth before that, and to his Resurrection. What happened is part of a longer story that involves all humanity and the chosen people in particular, and it is part of a more-than-human setting that involves Father, Son and Holy Spirit.
I simply want to stress how important it is not to put the crucifixion aside now that we live in the radiance of Easter. The Passion and all it means is not a past event only for our memories. The saving event keeps its link to Easter and afterwards. The Eucharist is a sacrifice because it makes present the sacrifice of the cross, is its memorial, and applies its fruits.
Jesus Christ will not be put to death again, there will be no repetition of what he did on earth some two thousand years ago. He is risen. Yet our experience and our understanding as Easter people include the road to Calvary and the Crucifixion. We believe in life everlasting and we rejoice in being forgiven, yet we still have to die and we still need to repent.
For us, the goodness of Good Friday is redemptive and transforming. Because of Christ's Passion and Resurrection we, who share in these saving mysteries, are not mortal or sinners the way we would have been had Christ not gone to Calvary and beyond.
The Passion, and the crucifixes that make Good Friday visible, are still needed by us who have to repent into a new kind of life and come to die. Because of what happened, death and sin are not what they were. Yet for us they remain unfinished struggles, ahead of us for as long as we live. We are still implicated in mortality and sin, yet share in a goodness that is gift from God and makes all the difference.