How could this happen to my husband?

My husband and I married almost 10 beautiful lovely years with 3 kids. We been trying to become pregnant again with baby number 4 but no luck for years now and we both check our self and when to the infertility doctor's. We found out my husband have NO SEMEN at all!! How is that possible? I ask the doctor? My husband already made me pregnant 3 times. The doctor was shocked and surprise and said very it's rare this happen to ur husband. There is no semen and no chance for pregnancy. He suggested us to do either sparm donor or adopted. My question is did anyone experience something like this?