Most scientists and marine biologists believe it died out around half a million years ago inbetween ice ages. It had been so successful that it had eaten itself out of existence. However, there is one fly in the ointment, the 'CHallenger tooth'w hich was found by the HMS Challenger, a RN ship, in about 1866 whilst it was dredging the sea bed for various items and shells, and brought up a megladon tooth (although it was not known to belong to the Megladon back then). This 'Challenger tooth' has been carbon-dated from being only 10,000 B>C., whilst the last Ice Age was beginning to end and that man was around, lived in caves and was starting to fish the sea - so it is conceivable that man encountered the Megladon at this time. Assuming the carbon-dating is genuine as it has been alleged that the tooth,w hilst genuine, was contaminated by other items around it and also in the clumsy way that it was obtained.
I do not know if the Megladon still exists, but I do know very little of the sea especially the deep parts of the globe have been properly investigated and explored. If it is down there, it could easily hide. Nobody has seen a truly gigantic squid but we do know that they can easily grow up to 90 feet long, probably bigger. We know this because a dead juvenile specimin was washed up off the New Zealand coast in the 19th century, it was a juvenile so marine biologists can compute how big it would have been had it lived to adult-hood. We can also tell the truly gigantic squid exists as its sucker marks from its tentacles have been found on the bodies of sperm whales that it has done battle with - they see from the size of the marks/welts just how big the squids can truly get.