On 31 December 1999, companies and organizations had to fix a problem with the computers. A bug, the millennium bug.
"In computer programs, the practice of representing the year with two digits becomes problematic with logical error(s) arising upon "rollover" from x99 to x00. This has caused some date-related processing to operate incorrectly for dates and times on and after January 1, 2000 and on other critical dates which were billed "event horizons". Without corrective action, it was suggested that long-working systems would break down when the "…97, 98, 99, 00…" ascending numbering assumption suddenly became invalid. Companies and organizations worldwide checked, fixed, and upgraded their computer systems." - From here.
Many people were scared about this. Imagine if all the computers of the world fail! Everything would stop. It'd be a catastrophe. Fortunately, just a few computers failed and the problem was fixed.
"The computer industry as a group, with a sense of social responsibility, quickly realized the huge problem that could result from their worst fears becoming a reality. Vital equipment such as various hospital paraphernalia, bank software, and national defense systems, and high speed Internet Service Providers were all operated by computers and the consequences of these systems crashing at once would be catastrophic. Hackers and those who purposely created mayhem took advantage of public panic and spread rumors about creating harmful computer viruses that would get into personal computers and then disrupt efficiency once the year 2000 New Year rang in." - From here.
I hope this helped.