If we default to "WTO" rules, then any imports from the EU would be subject to the same checks and regulations as imports from elsewhere in the world. This will cause massive delays and backlogs at ports (for stuff going in and out).
We import loads of essential stuff- like medicines, food, water purification chemicals, raw materials for basic necessities such as toilet paper and so on. Estimates seem to suggest we'd be importing about 10% of our current daily capacity due to these delays. This will lead to all sorts of shortages and problems.
There are already lots of people struggling to afford food in the UK for all sorts of reasons. If prices go up or there are shortages, some of these people will be very hungry indeed. Even people with money will face problems if there is nothing to buy in their local shops. This has the potential to lead to a lot of crime and civil unrest.
Long term, I'm sure we would find solutions, but how long are people actually willing to deal with food shortages for?
Meanwhile, lots of businesses rely on free trade and freedom of movement at the moment. If a business making cars in the UK uses parts from Europe and then exports some of the cars back to Europe to sell- well, after Brexit it will just be easier to have a factory in the EU and bring whatever cars they can sell into the UK, meaning job losses.
Loads of industries, from farming, to horse racing, to manufacturing, to financial services, to pharmaceuticals rely on the easy movement of goods, services, people and livestock between the UK and rEU. Some of these businesses- perhaps just the smaller ones at first with less of a financial buffer, will go under. That's lots of people losing their livelihoods. Multi-nationals may well decide it makes more sense to move production or offices or whatever to other countries. That's more job losses and loss of tax revenue for the government.
If people are losing their jobs at a time of price rises and potential shortages of some goods, that's pretty problematic for all sorts of reasons.
If we no deal, some people will lose their jobs, and due to that will lose their homes. There will be a higher proportion of pretty desperate people around. I'm sure anyone with half a brain can work out what the consequences of that will be...