How do I know what caused the head gasket to blow?

What is your question?

Had the transmission rebuilt and Jeep was running well. Filled up at gas station one day and up to that point no issues. Started Jeep and it would not stay running. When I tried to give it gas on and off in the neutral and/or park it was very rough and would die. Limped home approx 24 miles, restarting and coasting.Had it checked for the catalytic converter which was fine. Now, I am told that there is no compression in #2, the head gasket needs replaced and valve machined down. I want to know what caused this and what is a reasonable repair cost. Thank you,