abrupt collapse
One day you might wake up and the internet is gone. Power might be gone, TV and radio stations are offline. No one knows why or how. Stores will be overrun and goods necessary for survival such as food, water, hardware supplies, medicine and gasoline will be sold out. People will be out on the streets and observing anyone’s actions every day in great detail.
After 1-2 weeks, the government hands out emergency food reserves. After at most 4 weeks those reserves will be exhausted. Supply chains might be restored at this point to some extent. However the time investment, risks and money needed to obtain food will increase immediately and dramatically. For example in order to be handed out a day’s ration of food, every day you might be required to stand several hours in line and you will not be allowed to take more than for yourself. On your way home you might get robbed. This situation might worsen with time, such that obtaining food becomes too impractical and dangerous. People are beginning to starve after 3-6 months.
In urban areas, people will organize into mobs and gangs to steal from each other. It is difficult to predict how much this will be an issue. We know from historic examples, such as The Great Chinese Famine in 1958, that the police and military will not entirely break down and be able to maintain law and order to some degree. On the other hand, population density was much much lower back then, and high density is the main driver for mob and gang activity. Depending on where you live, ordinary neighbors could take up arms, team up to knock door by door and check homes for hoarded food, given that the situation for them was desperate enough. In any case, crime of any nature will increase several fold. Looting, theft, violence and burglary will become a common everyday occurrence, and walking alone outside will be much more dangerous. People will gravitate towards teaming up by family, ethnicity, religion or nationality and manufacture reasons to mistreat people outside of their group. Family and friends might betray you given that desperation or incentive is high enough.
The Great Chinese Famine, purely driven by government mismanagement, reduced the population by about 10% within two years. How many people will starve to death in what time frame is hard to predict and depends on your individual country’s resources. However some people guesstimate it to be around 40-60% within a year, some even as high as 90%.
gradual slow collapse
The main difference in a slow collapse would be prolonged ambiguity of the situation, and hence uncertainty how to react to it. It might manifest at first as a recession or economic crisis, or partial and sporadic dysfunctions of economic and communication systems, maybe mixed with wars, eventually leading to the same outcome. This might continue for several years. Prices will make life unsustainable rather slowly over time.
The upside of this is, that the threats and problems are a lot less imminent and extreme. On the other hand by prolonging the situation, your preparations don’t give you as much of an advantage to outlive the people who are unprepared, particularly so if you are poor to begin with. You might lose your job, car or home long before starvation becomes widespread. You have to expect, that the actions of other people against you will become more intelligent and organized over time. Also an ambiguous situation might seduce you to make poor long-term decisions. For example you might be inclined to live off your supplies without restocking them, supposedly only for a short while, rather than paying 2x or 5x the food prices that you are used to. Or you might feel that it is necessary to pay a lot of money on insulin and medicine for your sick relatives, or on pet food for your dog, rather than eating your dog. You might feel inclined to expose yourself as a prepper and share your supplies, if there is hope that the situation might end very soon and not continue forever.
The only solution to this problem is to act with foresight and reason devoid of emotion, and to make tough choices even when they are not really necessary at the time.