Where does our energy go after we die?
Well, as of currently there are a multitude of ways we can say this occurs.
Traditionally, when humans have died, they have been buried in the ground. The process here is very similar to when animals and plants die. When they do die, they provide a nutrient rich organism for which much smaller organisms feed off of, these organisms can range from microorganisms like bacteria and fungi to insects like beetles or woodlice to large creatures like vultures, all of which help in the decomposition of an organism.
When these decomposers eat the dead organism, they unlock the energy stored in it and digest it, this is the same which goes for when we eat chicken or potato, it is dead, and we are getting the nutrients and energy stored up in it. This energy can be stored in fats or sugars in the food, and we have the same.
These organisms will then use the nutrients they need from what they have consumed, and effectively- poo- out what they don't. It is these nutrients which are either eaten by microorganisms or are taken in by plants.
When we think of the Egyptians though, they have well-preserved dead, which them embalm. The process of embalming helps to prevent decay by stopping microorganisms from being able eat the embalmed mummy, this is due to the fact the moisture from the body is gone, and cover the body with resin to keep this moisture out as well as oxygen. By doing so it prevents the microorganisms reproducing due to the harsh conditions produced by embalming.
If we are not buried however, we may be cremated, this is like when we burn trees, coal, oil and natural gas as fuel. When we do combustion, we are releasing the stored up energy in it, and are breaking the bonds between the molecules. When we find ash, this is where incomplete combustion has occurred, but we however do not try to combust this.
These are the two typical ways we treat the body after we die, and how we lose energy just like everything else. It is a strange idea, as we humans do put ourselves in a place of 'high respect' and we have become so separated from the natural order that we forget that these processes still occur.
Great question, as it is very unusual and thought provoking, but nonetheless had an answer.
Hope this helps!