This probably doesnt really add anything, but I just want to reiterate, THANK YOU. You did something that is incredibly hard both physically and emotionally. You kept your cool and did what needed to be done, and what each of us hope that someone would do for us or our loved ones. The causes of suddenly death make a long list, and include things like congenital or acquired arrhythmias, vascular issues including aneurysm rupture or clot in the brain or lungs, and among many many others drug overdose which is sadly probably the leading cause in this age group these days. Im sorry that we cant really give you an answer here. As others have said, please seek help if you need it to process this event. Its a trauma and not something that many people have experienced. Remember that you did everything right, there is likely little that would have changed the outcome, and that you helped give his family a week to say goodbye.
I'm not a doctor so I can't say much on what happened but I went through something similar.An elderly man that walked past my house every day to go to church collapsed in front of my house. I have no idea how long he was there. But I heard a woman screaming for help so I went outside to look. He had no pulse. I gave him chest compressions until ambulance arrived but i knew he was already gone. I could hear air going in and out of his lungs from the chest compressions. I'll never forget that sound and how sad I felt doing it to him when I knew he was gone and feeling his ribs crack. This was 5+ years ago and I still remember it like it was yesterday. I still wonder if i had looked out my window sooner if he would have survived and wonder how long he was there before the woman saw him. I also remember how the fire fighters arrived first and stood around me and told me to keep going rather than take over. It was definitely traumatic.I feel so sorry for your neighbors family. So young and so much life ahead to live.Life really sucks sometimes.I will always remember the experience and it was hard to process at the time but it did get better as time went on.