How can an autopsy result come back negative?

I know someone who has a friend who died this summer. Police investigated the cause of death, nothing suspicious. Toxicology results were negative and they didn't find anything. She was in her mid 50s and very healthy as far as I know. How can a person just die suddenly and not give an answer? If a person dies, that means something happened or something's wrong, right? Could something have been missed even though this was investigated for months? Why would the cause of death be undetermined if the person appearently died for a reason?