"Year 2000 problem" was considered a millennium bug in computer program codes, where the year in date format was taken only as two digit. MM/DD/YY. So after Dec 31, 1999, the date format would read Jan 01, 2000 as 01/01/00.
Computer programs were designed to cut short 4-digit year as 2-digit in order to save memory space.
The Y2K bug created havoc among the world nations in the late 1990s, as the computer failure would cause major areas like banking, government operations, insurance etc would halt.
Not only the computers running on software programs but all the devices containing computer chips like elevators, medical equipment were believed to be at risk around the world.
Nearly $300 Billion was spent to upgrade the application programs accordingly.
The collective effort was a success and Jan 01, 2000 was born with a relief.
There was accusations that the threat was exaggerated from the beginning. But the computer experts proved the destructive effects of Y2K bug was real and rectified with utmost effort.