I am so very sorry your dog passed away, and you have no answers as to why.
It would be possible to do a Necropsy (Autopsy) on your dog's remains if it is only a couple of days or so since burial, but that will depend on temperature outside, moisture in the ground, etc. If it is mild or warm, and damp/wet, there will be accelerated decay. The drier and cooler it is outside the better.
But nevertheless, it wil be an upsetting thing to have to do, and you will seriously need answers to consider doing it. Of course it will not bring your dear dog back, only answer questions you have.
You are referring to a Necropsy. I am no expert but it should be done as soon as possible. Some causes, such as poison, may be able to be detected. unless the pet was kept cool, tissue will start to break down quickly. Your best source for an answer would be to call your Vet. I am so sorry for your loss.