I was working with a large manufacturing company at the turn of the millennium. They had invested in IT early and broadly. By the late 90's, we had a couple hundred VAX/PDP/Alpha clusters and a couple dozen Windows NT-based systems, with 30 years worth of custom code from assembler to FORTRAN and Top-down to OOP. We had FMS Forms, Visual Basic using ODBC, Pathworks, and custom network stacks. COBOL and QNX, RSX with overlays, and DEC everything. We had DCS and other PLC-based systems and interfaces. Automation, both logical and physical. All of it supporting hundreds of millions of dollars worth of equipment and annual revenues.
Around '97 we began Y2K testing on our systems and uncovered the alarming fact that we stood to suffer a cascading failure which would shut down our operation for at least a couple of years, put 5k people out of work, and upend our market segment globally. This had more to do with the sensitive nature of our chemical processes and custom DCS controls, so I wouldn't call it representative of industry as a whole, but at least for us, Y2K was a real horror story. No planes falling out of the sky or satellites crashing to Earth, but it would have resulted in thousands of miles worth of seized pipelines, vessels, and precision machinery that would have to be cut out and totally replaced.
We had an IT team with roughly 60 professionals across the spectrum from telecom to applications, though, and all the code was ours. Still, by the time Y2K rolled around, we were all at home (but carrying pagers). Most of us had dedicated up to half a year doing testing, remediation, and mock runs. Just setting up the testing environments kept us busy for quite a while. As an IT professional, I can testify it was the best of times. There was a lot of hype, and much of it was undeserved, but I hope those coming after realize it wasn't all smoke and mirrors.