Many computer programs that were written decades before 2000 stored dates as 6 digit numbers (in the form "mmddyy"). It was standard practice to store dates in this form. The reasoning - why waste costly disk space to store the additional 2 characters which were going to be "19" anyway?

Many of these programs were still in use in the years leading up to 2000. The Y2K paranoia was a result of fear that programs that controlled many important services like electricity, water, banking and government could fail when January 1, 2000 rolled around. Would computer programs think that it was January 1, 1900 again? Would we lose some or all of the services (and more) listed above?

There was a concerted effort during the years leading up to 2000 to identify and remediate these Y2K issues. Programs, applications and systems that passed were given the designation, "Y2k compliant." As we all know, the disastrous scenarios depicted in TV shows like .... never transpired.

Note: I included a link to the Y2K TV movie. I chuckled when I read the words, "Spoiler alert!"

No one really knew what was going to happen when computers that were set with default "19**" for dates hit "20**". Internal clock functions were feared to go into an endless loop, causing systems, servers, and PC's to crash, affecting everything from the banking industry to air traffic controls to individual industry, shutting down the economy, gas stations, and anything that contained a chip at all, including cars, cash registers, the power grid, and office building controls. People thought that anything electronic would be affected, turning the world back to about 1810, until it was resolved. So people tried to stockpile food, water, and fuel.

Only a few people were reasonably sure nothing would happen, and they found it hard to make their voices heard among the panic in the media.