The Medieval Era is a tough one to date. Historians will always argue over this. I like to call the time from roughly 500AD through 1500AD the Middle Ages (Dictionaries define "Medieval" as Relating or belonging to the Middle Ages.) Historians have marked the Medieval age as the end of the fifth century with the final fall of Rome (timing concurrent with evidence of widespread Christianity gaining a foothold in Europe), or 800AD (Charlemange's crowning as Holy Roman Emperor). Some dates that historians use to mark the end of this era include 1452 (invention of the printing press), 1485 (the fall of the last Plantagenet King of England: Richard III), and 1492 with the discovery of the Americas. All events (and more) undoubtedly contributed to the opening and closing of this rich and exciting era.
For purposes of genre fiction, Medieval is easier to pinpoint. Usually medieval romance is set in England so English-centric dates are used: 1100-1485. Prior to 1100 is also Medieval, but sub-subgenred as Norman or Saxon, and are less prevalent in publishing than the era of pageantry: 1100-1485. 1100 marks the beginning of the reign of Henry I (who was, yes, Norman) and 1485 (discussed above) which saw the dawn of the reign of the first Tudor King.
Some other wonderful writers who set their stories in the Medieval era are:
Happy reading, Samantha James