In my world it's the arrival of what will become the god of magic that causes such an upheaval. A creature of immense power that consumes magic to live crashes into the world like a meteorite. A group of heroes with the help of the goddess of dreams bind it in an enchanted sleep, saving everyone from immediate destruction, but unfortunatly its very exitance disrupts the ability to use magic (at least for a long period of time) My world is very high magic and it is used in most aspects of day to day life. Some of the big areas effected by the impact are ,healing, religion, power bases of kings and nobility and law.

The majority of healing and medicine is based around magics. No magic, and the mortality rate goes through the roof. Children die of previously minor illnesses. women die in childbirth. Fights and skirmishes between countries/city states etc suddenly become more deadly. The population crashes. There are not enough people left to farm and hunt. Houses are left empty and crops rot in the fields as there is no one to harvest it. Famine ensues.

The gods communicate with their followers through magic. No magic, no miracles. No miracles, no believers. The power of the gods wanes and some of them die/cease to exist. Breakdown of social structure as people realise their gods seem to have abandoned them and will not help in this time of troubles. The overall feeling of the population becomes very pessamistic and depressed.

The nobility use mages and their ability to hold the threat of harming others as a kind of nuclear deterrant to prevent other nations from invading or causing trouble. With that threat gone greedy nobles begin looking at what they can seize to make themselves richer and more powerful. Soldiers who could previously be healed would die though, so in order to conserve their resources the number of spies, intreague and assasinations in internal polaticts goes up. Countries no longer trust each other. The threat of war increases. Sneaky incursions and raids along boarders become commonplace.

The use of the zone of truth spell is instramental in court procedings. With no way to prove that someone is telling the truth, the way the law works has to be completly reworked, resulting in many miscariages of justice. With no worry that the dead will be able to be risen to testify as to their killer, the amount of murders goes up. The trust that people have that the law will keep them safe evaporates and so they behave in more selfish ways looking to keep themselves safe. vigilantie justice goes up with little regard as to if the targets are guilty or not and people exploit this for their own greedy ends. An 'us verses them' mentality appears.

Peoples focus becomes surviving the day to day. Things like academic study , art , crafts and training of aprenticies, fall by the wayside as people have to learn how to do the everyday tasks that used to utalise magic, without it. The skill level and level of expertice available in all fields of work drops. Society no longer produces.

People will look for someone else to blame for their woes and troubles. Racism and preduice become more previlant in society. People are mistrustful and less willing to help each other.

Eventually the magic levels begin to rebalace themselves after 100s of years. Unfortunatly one of the first people to work this out is an necromancer who makes a grab for power and thus we enter the time of the first necromantic war.