Is there an international regulatory body for investigating UAV accidents? Does it exist on any national level?
This question is rather broad, and, as worded, does not admit to a good answer beyond "no" to the first part, and "yes" to the second. That said, the rest of this answer might be useful to you.
There is no international body to investigate UAV accidents, just as there is no international body to investigate ordinary aircraft accidents. There are a great many national bodies.
In the United States, the National Transportation Safety Board is responsible for accident investigations, however, in order to get involved they would have to both know about the accident, and consider it worth investigating. The FAA requires that pilots operating under Part 107 report any accidents involving serious injury, or property damage greater than $500 (excluding the UAV) to the FAA. Presumably they would do some form of investigation to assess the remote pilot's continued suitability to hold a license. Hobbyists don't fly under part 107, however.
Other countries have their own investigation groups, such as the Air Accidents Investigation Branch in the United Kingdom. Wikipedia has a list of organizations that investigate aircraft accidents and incidents, but I have no idea how complete it is, and you would have to check each one to know what their particular policies are.