Clarke thought what would likely precipitate the process of collapse would be some sort of global catastrophe. It could be   another pandemic for which no vaccine can be found such as one bioengineered. That is a very credible possibility because some viruses mutate much faster than others. The average virus mutates about 1 million times faster than the human genome but there are viruses that mutate HUNDREDS of times faster than that. 

An example of a virus that mutates much faster than say one of the novel coronaviruses that have come out of the PRC over the last century and a half, is HIV. Corona viruses cause the Wuhan Flu, but the virus that causes AIDS is a different kind called a retrovirus. A retrovirus could possibly serve as a template for some asshole to create a virus for which no vaccine can be made. Retroviruses and rhinoviruses mutate so fast, that by the time a vaccine is created for the dominant strain, it will have already mutated into another strain.* 

The problem with vaccines is that they are very much strain-specific. IOW, each vaccine must be tailored to the specific strain of virus targeted or it won't work. The reason no vaccine has ever been created for the common cold or AIDS is that the viruses that cause those diseases mutate so fast, that by the time a vaccine is generated, the virus has already mutated to an entirely different genome for which the vaccine is worthless. 

That's why no vaccine has ever been found for either HIV or the common cold. 

Eventually some bright lunatic may well be able to figure out how to use gain-of-function to modify some retrovirus or rhinovirus and release it into the world. Imagine a virus as contagious as the common cold but as deadly as AIDS (AIDS has a roughly 50% fatality rate, and it would be much higher without anti-viral drugs).

That would be truly catastrophic because the result would be very much like the result of the other great plagues in the era before modern vaccines existed. The Bubonic Plague wiped out 1/3 of the population of Europe. The Spanish Flu (which was actually originally started by Chinese workers living in Spain) killed possibly as many as 50 million people. Spanish Flu had only a 10% fatality rate and infected roughly half a billion people. 

In Clarke's scenario, a trigger event like a plague is just the first step in the process of total global collapse. After the plague hit, nations on the brink of collapse due to inability to support their populations biological needs, would experience total internal collapse. They would swiftly degenerate into military oligarchies - survival needs would doubtless drive some nations with thermonuclear weapons to use them on neighbors to prevent them from plundering their own dwindling supplies. 

So the result would be a plague-ridden world without hope of a vaccine, which was increasingly violent, lawless and without infrastructure to supply the biological needs of the world's population. 

Clarke thought that the likely result of this scenario would be the technological paralysis of the Human Race for at least two or three centuries. No technology would be developed, no scientific progress at all. Science would grind to a halt because there would be no funding or infrastructure to support research.  

In addition to that, obviously no progress would be possible on a global asteroid defense (which we are just now taking the first baby steps toward creating), and we are already in a desperately perilous situation in that regard. See, if we detected a ELE object (6 kilometers or larger of solid material  @ 35,000 MPh velocity) today that had an impact ETA less than 25 years away, it is extremely unlikely with existing technology, that we could divert it in time. In that event the destruction would be total - the human race would likely become extinct. 

* The variants of Covid-19 have been fairly simple and therefore either killed by vaccines for other variants or fairly easy to build a new vaccine to combat. The strains that emerge from HIV or Rhinoviruses are so complex that vaccines not specifically targeting the precise strains are useless.