This is the second time in about a month that my Dyson DC 40 has shut down from overheating. The symptoms start with loss of suction from the head assembly (leaving dust bunny trails, etc.) while the hose end still had good suction.
I started out by disassembling the head unit and cleaning out the long dog hairs there, but there wasn't any change in the amount of suction when vacuuming carpet and dust bunnies were still being left behind.
I also made sure to empty the canister of all of the collected dog hair, but there was no change in suction. Finally, I decided to remove the cover on the HEPA filter to let it cool down.
Once disconnected, I more clearly saw the hose connection that clogged the last time, and sure enough, a combo of dog hair and something perfectly sized to get wedged and stuck had completely blocked off the flow of air.
Unfortunately, now the vacuum cleaner has shut down until it resets and cools down. Maybe Dyson would like to study the special properties of our dog's hair?
Update: Leaving the cover open and the vacuum unplugged for about 30-45 minutes brought it back to life.
Update (February 25, 2018, one week after this post): The DC 40 stopped working. We checked all the clog points, cleared the filters, unplugged, let things cool down, and replugged... It never came back. We ended up buying the Shark Rotator NV650 at our local Walmart instead. The price was only $20 more than my refurbished DC 40 cost, and is much quieter and more portable.