You're probably getting down voted because you asked here instead of a search engine, and many people think it's common knowledge, and it was already answered in this thread.
Sometimes an innocent question looks like someone JAQing off.
It was a massive threat as it would break banking records and aircraft flight paths. Those industries spent millions to fix the problem. In 14 years(2038) we'll have a similar problem with all 32bit computers breaking if they haven't had firmware updates to store UTC time as a 64bit number composed of two 32bit numbers. Lots of medical, industrial, and government equipment will need to either be patched or replaced.
By comparison, there were a few systems that had issues on February 29th because of leap day. Issues with such a routine thing in this current day should be unthinkable.
There wasn't much of a real "threat", in that planes wouldn't fall out of the sky. but banking systems would probably get quite confused, and potentially lead to people being unable to access money easily until it got fixed.