Germany
Interregnum
Interregnum is Latin; it means "between kings" and refers to any period when there is no legitimate king in power. Technically, there was no interregnum because at least someone laid claim to the title, but from 1250 to 1273 the Empire had no effective ruler, and from 1254 it had multiple claimants. The lack of authority was so pronounced that the era has long been known to historians as the Interregnum, capitalization and all. There were many interregna during the Middle Ages, but say "the Interregnum" and medievalists will assume you mean this one.
The worst effects were about what you might guess: the work done by three generations of Hohentaufen (Frederick I, Henry VI, Frederick II) was undone in a single generation. Sicily, Italy, and Germany went their separate ways and would never even come close to being united under a single monarch. In each area, the local powers (princes in Germany, cities in Italy, barons in Sicily) escaped royal control. There would eventually be a strong king in Sicily and a weak emperor in Germany, while in northern Italy the cities themselves would dominate.