In Asimov's Foundation series, the Empire's nuclear power stations were controlled using vacuum tubes and relays.
Out-of-universe, this was because solid-state transistors had not been invented when the story was written. In-universe, this might have been for lack of solid-state transistor technology, or because vacuum tubes and relays are easier to understand, make, and replace than bits of integrated circuits. Vacuum tubes and relays can also be designed to handle much larger currents, voltages, and power flows than integrated circuits.
In-universe, the Empire lost the technology to be able to replace even "a single quartz D-tube". If enough of those essentially-glass vacuum tubes were physically smashed, an entire nuclear power station would be rendered out-of-commission.
In real life, expensive vacuum tubes have cathodes with a limited life. For example, the travelling wave tubes on weather and communications satellites typically have a design life of 12 or 15 years. The limiting factor is the amount of metal in the cathode. (This does not have to be the limiting factor. Some tubes are predicted to have cathode lives of hundreds of years. But satellite parts have strict mass budgets. Every tenth of a gram counts, so only a modest amount of extra cathode metal is included.) When the tube is operating, the cathode is heated to a high temperature, and the metal slowly evaporates at a very predictable rate. The satellite will need to be decommissioned or replaced before the metal's reservoir runs out.
The vacuum tubes in the control system of the original poster's power plant were engineered to have a consistently long life. But eventually the cathodes will run out of metal, and the vacuum tubes will stop working. Many vacuum tubes will have experienced similar times at operating temperature, so that they fail after a similar length of time in all of the power plants. As gbjbaanb pointed out in a comment, the consistently long life does not need to be 2,000 years. "All you need is a box of replacements and instructions of what to do to fit them... after 2,000 years the priest-tech goes to find a new one and discovers the box of spares is empty. And, over the same century - all plants had only a similar number of repalcement parts, so will all fail around the same time, particularly if parts were shared out." This scenario is especially likely if the last vacuum tube factory was asked to make a large number of spares just before it went out of business; this would have both extended the life of the factory by a few years, and made it much more difficult to start a business to replace the factory.
In real life, the processes to make cathodes for high-efficiency vacuum tubes are notoriously finicky. It is very easy to accidentally contaminate the cathode material while manufacturing it. This is part of why there are so few companies capable of making space-rated vacuum tubes, and why the number of such companies is decreasing.
In the original poster's scenario, the vacuum tube manufacturers could well have been out of business for centuries. A great deal of technology would need to be reinvented to replace the failed vacuum tubes. Or the characters could try to get by with much more primitive vacuum tubes, and risk reliability problems in their nuclear power plants' control systems. For that matter, the original vacuum tubes might be much more space-efficient and energy-efficient. There might not be enough room or available "conditioned" power to install a jury-rigged control system.
To get an idea of how much technology might need to be reinvented, Chapter 5 (pages 47 - 102) of Scott Gilmour's Principles of Traveling Wave Tubes provides an overview of "Cathodes", with 30 scientific references. Some of the other chapters discuss "Electron Guns", "Reliability", and methods for modulating / using the outputs from "Electron Guns".