If you want the PatBag to com back after death, execute this Command: /trigger respawn at the Spawn.
To Reset the PetBag execute /trigger respawn again. (It will kill all PetBag's and the Respawn-Point)
You can only run the Command in the Creative Mode.
Do you always want a Back Pack in Minecraft? These is a Chest which works like a Dog. It flollows you anywere.