There are two that really scare me. Not the 'keep me up at night' scare, but 'wow that would be terrible' scare.
First, the errant solar flare or similar stellar event. One good blast turns all our satellites into space junk, and render most of the longer planetside cabling useless. We wouldn't be stone age, but in a major event millions could die and it would take decades to recover the infrastructure.
Second, SCATA controls really scare me. I spent several years working on government infrastructure projects. ALMOST NOTHING is secure. Imagine if all the bad guys -- be they large governments or tiny coordinated attackers -- hit the computer controls all at once. The two biggest are electricity and water. Probably only thousands or tens of thousands of deaths with that, and a few years to recover.
There have been a few major blackouts that caused problems. Depending on your location (Sorry, US centric) you might remember:
The 2011 blackout in Southern California only lasted a day for most people but caused a few hundred million dollars in damages, and resulted in contaminated water to millions of people. Some industries had chemical releases that caused assorted contamination and problems. Transportation and other systems struggled, although apart from a massive traffic jam things mostly worked out okay.
The 2003 blackout in the Northeastern US hit a similar geographic region but about 8x the number of people. Transportation shut down, water and sewage was a serious problem for days after power returned. Major sewage spills and sewage treatment plant overflows, chemical releases like the Ontario chemical plant, and a lack of water pressure in the cities caused messes that took months to clean up.
In both events power was initially restored to key areas within about six hours, with the bulk of it restored within two days. Just those few hours caused serious problems, lots of contamination, and even a few deaths. Yet they were short enough that people still had fuel in their vehicles, food stocked in refrigerators and coolers, and only a few retail stores were looted for goods.
But with a massive solar event or with massive coordinated SCADA-induced equipment damage, it could be days, weeks, or even months before power is restored broadly, and the effect could be global rather than regional. Within a few days most city-dwellers will be out of fuel. An enormous number of the population only keep enough food in their homes for a few weeks, and the food distribution networks would be inoperative. Communications networks would go down, distribution networks would go down. Those living in harsh weather -- perhaps under heavy snowfall or summer heat -- could have secondary issues from that.
And it wouldn't be regional. The solar event could take out half of the world geographically, or maybe more for a long event. It's a situation I'd rather wake up to and hear on the news that the other side of the world, not mine, was destroyed.
DNS getting shut down? Not too big of a deal. Word of mouth and other networks would spread common DNS numbers, and individual ISPs could get alternatives in place readily enough.