If you want to take it in for a necropsy, don't freeze it!
Put it in the fridge!
Stephanie, the term necropsy is generally used for non human postmortem examination. They usually only use the term autopsies on humans.
OP, I'm sorry this happened to you. Sometimes, some animals just do not do as well and don't make it. This sounds like a death you could not control, it's not your fault.