what does collapse look at you? People dying in mass? To many, civilization collapse means the collapse of advanced 'civic' functions, from trade, to order, to normal civic functions (regulations, laws and order, registrations/admmin functions, coinage etc).

It is replaced by a completely archaic/primitive way of living, with petty kindoms, local warlords, and more.

Also, in parts of europe, whole populations got ravaged by the justinian plague that came later, and eventually new people moved in as mass migration, somme cocqureing to completely replace the previous population. (e.g south slavs in the balkans). their culture, was much more aggrarian and primitive commpared to the one they did replace. Cities fell in ruins and disrepari, aquaducts stopped working, and whole thriving towns and citiess turned into piles of ruble or forgoten ruins.

This process took about 200 years and it didn't happen overnight. (470ad to 670ad), but the decline was remarkable.

So, yeah, 'ciivilization' collaped, to be replaced by a more archaic primitive way of living. It is like rolling the clock back few hundreds of years of progress and going back to 500bc. way of living.

except a great deal of technology was lost.

One of Hobbs insights is that in an era with a non-functional state, the rewards of technological investment are few. Also loss of state capacity has severe impact on international trade; recognize that trade also brings ideas and innovation. And any technology that relies on supply chains becomes challenging, any technology that relies on high levels of education becomes challenging.