I'm not sure if this counts or not, since it wasn't a monarchy, the successor actually thought the ruler was dead initially (as a result of the detonation of the bomb that he had planted,) and it also wasn't in the Middle Ages, but Colonel Claus von Stauffenberg comes to mind.
In what was probably the closest an assassination plot came to succeeding against Hitler, Stauffenberg planted a bomb in a meeting he attended with Hitler in 1944 and then left the meeting due to receiving a planned telephone call. The device did detonate, at which point Stauffenberg assumed Hitler was dead and activated a plan known as Operation Valkyrie, which allowed Stauffenberg and his accomplices to briefly take over control of most of the German government.
Unfortunately, the conference took place in an above-ground conference room instead of the normal underground bunker due to the weather, so the pressure from the blast was not contained within the room and, thus, was not as deadly. Hitler was injured, but survived. Since the plot was already underway when he found out that Hitler had survived, Stauffenberg pressed for it to continue and attempted to deceive others into believing that Hitler was, in fact, dead.
However, since Hitler was not actually dead, the plot began to fall apart within a few hours as news slowly spread of Hitler's survival. Ultimately, the plot failed and Stauffenberg was executed about 12 hours after the detonation of the bomb. But, for a few hours, he and his accomplices did control much of Nazi Germany due to making people think Hitler was dead.
This coup attempt was the plot of the 2008 movie Valkyrie starring Tom Cruise.