Apparently all of the most powerful ancient kingdoms and empires died in the same short time period, and it took hundreds of years for them to come back. Is there any concrete explanation for this?
No one knows for sure, but there are plenty of theories -- drought, population shifts/migration, climate change, earthquake "storms," the infamous "Sea Peoples," or a general systems collapsed (brought about by a combination of several of these factors. Sort of a "chain reaction of events" theory).
I can expand on some of these a little if you'd like (I'm an undergrad who's studied this topic), but I'd really recommend you look into books written by Dr. Eric H. Cline, whom I've studied with for years -- he's an expert on the Late Bronze Age and a best-selling author. He literally just published a new book on this topic, 1177: The Year Civilization Collapsed, so if you're really interested in this subject, I'd highly recommend you pick it up.