Interested to know how does a hernia turn to gangrene?

The contents die: You are referring to a strangulated hernia. This is when the contents of the hernia lose their blood supply and become gangrenous.

Here is...: Gangrene is the ending stage of tissue dying of complete deprivation of blood circulation by many causes with the hernia leading the pack. Hernia results from any tissue sliding and slipping into an opening with inability to slide back to its original location, that is, spontaneous reducible; among them, inguinal hernia is the most common. In the early stage, most inguinal hernia can be reduced.