Found dead baby birds

Hi All 

Today I was so excited as I heard baby birds in a nest under my eaves.  It was lovely to hear them and I was hoping I would be able to watch them grow.  At the time I stood close to the house to listen to them, the nest was well hidden and about three feet higher than my head. 

Tonight I just walk past the nest and found three dead chicks on the ground.  

Can you tell me why this might have happened.  I  don’t hear any chicks in the nest now!!!!!!