That kind of temperature drop can kill them, but it all depends on how long it was for and how far into incubation they were at the time. They are much more sensitive to temperature drops early on in incubation.
When you candle, can you see any movement at all around the edge of the air cell? You should be able to see the dark mass pushing on the edges of the air cell if they are still alive in there. 28 days is just a ballpark, so they might still be ok. Low temps during incubation can also slow them down so they hatch a little later as well. But, look closely at the edges of the air cell and see if you see any movement at all.