The line comes from the British sitcom The IT Crowd, which follows the basement IT department of a London company and its famously unhelpful support desk. Turning a device off and on again became the staff's default answer to almost every problem, turning a real piece of tech advice into a running joke across the series. Clips and soundboards later carried the phrase beyond the show, where it stuck because it describes a fix almost everyone has actually tried. It still works as shorthand for lazy, obvious tech support.