We often retreat when faced with tragedy. We desperately try to put our lives back to the way they were as though nothing had happened. But God always calls us forward. The changes that we experience in our lives are supposed to be opportunities for us to grow. This is shown through Peter when Christ asks him three times if Peter loves Him. Peter is distraught by the third question; he doesn't realize that Jesus is giving him a chance to redeem the three times that he denied knowing (let alone loving) Christ on the eve of the crucifixion.
The tragedy of Jesus' death and the subsequent glorious resurrection gave Peter a chance at redemption. This is true for all of us as well. With God there is never a point of no return; He is always ready to welcome us back, always ready to see us grow.