I've actually been peripherally involved in something like this, I think it was September 2000, maybe a few months later.
An ATC system where they had two networks, the "office" network and the "core business of actually directing flights" network. The offices were around 10 miles away from the airport, which is where the ATC mainframe and - obviously- the ATC tower were. There was a bridge between the networks.
There were building works going on in the city, and they cut the cables while excavating. The agreement with the provider was that there would be two high speed lines and that they would not be in the same bundles of cables (or maybe glass fibre, if that was a "thing" back then). Unfortunately they had not insisted in the cables being in the separate ditches. The excavator cut both connections.
This should not have been a problem, two separate networks right? The critical one was at the airport and the whole point of that setup was so things would carry on if the offices were cut off. Most of the network group worked in the offices rather than at the airport. Some component was in the offices, the router at the airport was set up so that the control traffic to that component had the highest priority.
Bottom line, the core network was nominally unaffected, but a router there was trying to reach the office network and it was using all of its processor power for that. The core network pretty much came to a standstill. I can't remember how long that outage lasted but it was several hours.
Lessons were learned and all that.
The offices moved to a new location a year or so later, something which had been in preparation for a few years by then. New offices, new cables, whatever.
In 2003 (or maybe early 2004) something similar happened, an excavator chopped the connection. I was working on the mainframes at the airport from the offices so obviously I was affected, I just drove to the airport and carried on from there, everything worked exactly as it was supposed to - the offices could communicate with each other and the ATC Tower had full access to the ATC mainframes. It took a few hours before the two networks were linked to each other again but nobody outside noticed anything.