Same thing as if everyone stopped paying any type of loans. A shock to the banking system, potentially a collapse if the debt in question is a significant percentage of all debt. Many people would lose their savings.
And no don't hope that bank owners would absorb the debt, they would just liquidate the bank in a bankruptcy wiping out everyone's deposits.

Edit: In most countries there's also a deposit insurance scheme meant to cover cases of bank failure. But it can cover one or two banks failing, not all of them at once.

Police enforcement. Like when the national guard was sent to break unions.

Not much. You can't spend enough of their money before they stop your spending and start collecting it.

How much credit can you get without a high income or a lot of assets? Hopefully much smaller than your mortgage (which is that the 2008 crisis was about).

Exactly what happens depends on law where you live. Here you'd get a court order: First confiscation of cash or cash equivalents. Second sale of assets, in the end your pay would be garnished. Since not everyone is forced to sell their house (like 2008) the price drop would be smaller but spread across the entire economy.

Unless you're willing to illegally work without a contract and only take cash payments they will get their money back.

Anyone who did not participate in it would have a good time buying cheap stuff in the forced sale.

Companies like Visa and similar would go out of business with no customers. The government might intervene with bailouts to stop banks from going down. Online shops would start accepting bank transfers (or Venmo in 'Murica) more universally. Crypto would have another hype cycle.

I don't really buy that it would implode the economy, since it runs on all kinds of more tangible things and there's other forms of currency and debt to fall back on, and I also don't buy that the government would seriously enforce against a strike if it had 100% participation - usually just a few percent is election-defining. At most I expect whining and a maybe a few impotent attempts to discourage it before they get the picture.

wage garnishment mostly

Believe it or not, straight to jail.

https://en.wikipedia.org/wiki/2007%E2%80%932008_financial_crisis

Hugh government bail out (American here). Taxpayer gets stuck with the bill. Companies loose nothing (probably make even more than way).

They would all get late fees.

The financial system would collapse, leading to unimaginable suffering until it is rebuilt again. Tens of millions of deaths in a few years in USA alone.