Clearly, some autocratic countries become democratic briefly, and then lapse back into some form of autocracy. The Weimar republic lasted from 1919 to 1933, for example, before Hitler effectively abrogated the constitution. However, I can't think of any case, in the modern era, of a multi-generational democracy that has ever reverted back. For example, Britain managed to lose an entire empire without ever any serious threat to its status as a democratic country. Britain and the US made it through two world wars and a great depression without losing their democratic status. Indeed the US managed to fight a civil war with itself, without either side actually giving up on the democratic form of governance.
So my question is this: what is the longest period that a country has been a democracy, and then reverted to some non-democratic form of government? Let's confine it to the post-industrial revolution era.