Welcome to HCM
It is shocking story and incident like that is also happened even my family when my four month nephew died at night without any illness(though our family has two child specialist).This is called SIDS or Sudden Infant Death Syndrome
.Till date no direct causes have been found only some possible factors are postulated.
Though I not 100% sure about your baby,s causes of death but this is the possible explanation.
Thanking you wish you a long life.