Cold blooded animals have several sets of enzymes that maintain bodily functions at different temperatures, while warm-blooded only need and have one set. This would be the first big barrier to evolving (back) to being cold blooded. Not having a high enough metabolism to sustain the brain at our intelligence levels would be the second barrier.
I don't think there are any circumstances that could force modern humans to evolve to being cold blooded. We're simply too good at changing our environment to suit our needs. With stone age or earlier humans, you might still have strong enough environmental pressure to direct evolution.
Some mammals hibernate during winter, greatly lowering their metabolism and conserving energy in times of food shortages. That is probably the best route towards evolving cold-bloodedness. Humans might still have some of those genes and they could lead to a lethargic state of semi-hibernation, which over a million years or so could be refined into a mostly functional state regardless of body temperature. The catch is that energy efficiency implies lower brain functions, making these humans dumber again.
Btw, cold-blooded animals also die at 40 C or higher (vital proteins start breaking down at 42-44 C for everyone) and freeze at 0 C (or a little below for marine animals with anti-freeze, as commented), so they have a narrower temperature range than warm blooded animals.