For a huge population reduction, I'd say unchecked global warming followed by an epidemic. However, our level of technology probably won't get reduced significantly. Experimental technologies such as the LHC might get lost, but going off Rome, we won't go back to farming peasantry. Some areas might lose central government but they'll just become several states. However, the collapse might be very different from Rome, as there aren't any barbarians we don't know about, barring actual ASBs. The American Empire might collapse like Rome, but not for a long while, and that wouldn't lead to a collapse of the west.
Most of the reversions to earlier technology in history were triggered by foreign invasion, and even if there was a major war, China vs. Russia, say, and China knocks down every building in Russia, the Russians won't return to feudal serfdom. Well, they might, but they'd be feudal serfs with guns, electricity, cars, and computers (maybe computers is a stretch, but I think the new dukes and lords would have them).