There are many things that Baba has lost. In view of these things, let me name a few of them.
First, Baba lost the country he loved as well as his city of Kabul. In chapter 15 when Amir goes back, he sees what has happened to the beautiful city of his youth. In connect with this, Baba also lost his house. Baba expected to return soon, but he never did. Rahim looked over this property.
Second, Baba more significantly, lost his identity as an important person in society. He was a respected person in the community back in Afganhistan. This is one of the great loses in the novel. There is an incident in a convenience store in California, which shows this point. Baba gets very angry when the store teller asked for ID. He was offended at the implications. What Baba has been reduced to is a gas station attendant in America, where he was an important member of society.