I’ve actually seen this happen right before me. Was working outside on a very hot day (over 100) and the guy beside me just drops like a rock. No warning, no nothing.
I immediately started chest compressions on him while somebody else called 911. They tried to jump start his heart but to no avail.
I had anxiety attacks over it for more than a year. Something you never forget.
You did fine. CPR does save lives, but only works about 5% of the time.