This is very serious. This means either the US or South Korea sank it or they are holding the vessel and possibly its crew and they are harboring prisoners of war!

Source

http://www.ibtimes.com/north-korea-submarine-missing-us-says-report-2335155

http://www.cnn.com/2016/03/11/politics/u-s-says-north-korean-submarine-missing/index.html