Causes of zero sperm count

Hi everyone, we also just found out that my husband has a zero sperm count. I am devastated and confused. Unfortunately the wait to see the urologist will be months. In the meantime, im just looking for some input. Has anyone gone through testing and found out why the count was zero and reversed it? Are the chances of him ever producing very slim? His father had issues which lead to low sperm count due to a blockage. I am not sure if a blockage could cause zero... any input would be greatly appreciated. Thank you ❤️