The abandoned nest

After returning from holiday I was very happy I see Robins nest under the roof. Every day, while exiting on the porch I secretly watched it.
On June 30th I noticed one eggshell dropped on the ground. The same day I spotted proud father carrying the first meal for his baby.
On July 1st the parents were taking turns on taking care of the new family. Today there is the sad ending to the beginnings of joy.
This nest is abandoned. Through the whole time I didn't open the window in my computer/guest room. I wouldn't go on garage roof to have better view of the nest, but despite that something went wrong :-(