What happened to the Roman temples and public buildings after the fall of the Roman Empire when they suddenly became redundant? The answer is simple: they were robbed for building materials.
Cathedral church of Santa Maria Maggiore, Civita Castellana: exterior Cosmati decoration (note the gold for added glitter)
Just think how much material there was to take! All over the Roman Empire there were thousands of temples, expensively clad in marble – some of it carved, and with marble or stone columns supporting pediments and roofs of dressed stone. And that was just the temples. Every Roman town would have had its forum, with colonnades, amphitheatre, public baths, and dozens of other public buildings, all made of expensively cut stone and marble.