Societal collapse is a catchall that covers something very serious and sudden, at one extreme; ranging through to a series of losses over a relatively long period of time.
At each loss we would see a decrease in well-being for some groups of people. And in general those people will be those with the least income, wealth and power. I refer to this process as societal crumbling.
<b>Societal Collapse</b>
Here's a scenario that might be described as serious and sudden:
A banking crisis such as we saw in 2008, but this time the governments fail in their bailouts, banks are over-extended and go into receivership, cash machines stop working. So people can only use the cash they have in their pockets or under their beds to buy their essentials. And likely at the same time there are shortages - and so price rises - of many essentials. People have to go without. Greek people experienced something like this in 2015 - see this Wired report 
.
And likely alongside this public services such as the NHS, education, policing, transport fail to deliver what's expected of them.
<b>Societal Crumbling</b>
Here are some scenarios that I would describe as societal crumbling:
<ul><li>Sea level rise resulting in a major, and perhaps permanent, relocation of people and factories</li>
<li>Continuing food price rises leading to hunger and unrest</li>
<li>Another pandemic, more debilitating than COVID</li>
<li>Significant drop in temperatures as a result of weakening of the Gulf Stream and AMOC (Atlantic Meridional Overturning Current); leading to increased energy leads and rising food prices</li></ul>
There are many other scenarios in this category. We cannot forecast which of these might occur first. And we can't rule out more than one happening at a time.
And as always, the powerless, the elderly and the poorest people will suffer the most. Many are likely to die, or become more cold and hungry. More are likely to be displaced to inadequate housing in areas with few shops and poor services.