There are lots of Stratum 0 servers out there; basically anything with an atomic clock will do. They all count seconds independently from one another, all slowly diverging over time, with offset intervals being measured by mutual synchronization using a number of means (how is this done is interesting all by itself). Some atomic clocks are more accurate than others, and an ensemble of these is typically regarded as 'the' master clock.

To quote the ITU: "UTC is based on about 450 atomic clocks, which are maintained in 85 national time laboratories around the world." https://www.itu.int/hub/2023/07/coordinated-universal-time-a...

Beyond this, as other commenters have said, anyone who is really dependent on having exact time (such as telcos, broadcasters, and those running global synchronized databases) should have their own atomic clock fleets. There are thousands and thousands of atomic clocks in these fleets worldwide. Moreover, GPS time, used by many to act as their time reference, is distributed by yet other means.

Nothing bad will happen, except to those who have deliberately made these specific Stratum 0 clocks their only reference time. Anyone who has either left their computer at its factory settings or has set up their NTP configuration in accordance to recommended settings will be unaffected by this.