Social security can 'run out', because it's setup with separate accounting. It's sort of designed that current year social security taxes pay for current year benefits ... in the 70s and 80s increases in taxes and decreases in benefits lead to a surplus of taxes collected which was held under the social security account. Since about 2009, income and costs have been pretty close and since about 2017, costs have consistently been more than income. 'social security runs out' when the accumulated funds have all been paid out.
If there's no other action, current law says benefits will be cut so that benefit payments don't exceed the tax income.
Closing social security off to new workers doesn't help, because current workers pay the bulk of current benefits.
There needs to either be additional funding (from general taxes or a rise in social security taxes) or a reduction in benefits. But nobody wants to do either of those, so chances are we'll get the default option.
On the plus side, I was a teen in the 1990s and my high school economics class suggested social security might not be wholy reliable, so we should separately save for retirement on our own. I estimate we'll have had at least 30 years of warning when benefits are cut, but likely many will still be taken by surprise, or will not have been able to prepare despite foreknowledge.