“To date, intentional computer-based attacks that have direct physical impacts have been few and far between, so far as we know. That doesn’t mean these scenarios couldn’t happen in real life, or that there aren’t real and serious vulnerabilities in the country’s critical infrastructure networks. There is a perception that we haven’t yet experienced such a catastrophe because of a combination of luck and the reluctance on the part of nations, militias and other entities capable of launching a cyber attack to set a dangerous precedent. In 2011, for instance, news outlets reported that the Obama administration decided against infiltrating the computer systems of the Libyan government to interfere with their military communications and air-defense system due to concerns about whether other nations might follow suit as well as uncertainty surrounding whether such measures required Congressional approval. The Stuxnet worm that in 2010 struck Iranian nuclear facilities, causing centrifuges to speed up, thereby interrupting the uranium enrichment process essential for the development of nuclear technology, is the exception, judging by unclassified knowledge.”
Source:How Would the U.S. Respond to a Nightmare Cyber Attack?: Scientific American