The optimum range of temperature is between 55 and 65 degrees. Higher, and the embryo is likely to begin developing. 71-72 degrees marks this point. Lower and the viable germ begins to degrade, with 45 and below considered to be the cutoff point.
However, there are untold numbers of people who will tell that they have incubated "too cold" eggs, or those stored in the fridge, successfully. The odds are not in favor of this and the viable hatches rate is probably going to be low. But with nature, variables are many and nothing is absolute.
I would suggest however, that eggs at or near freezing for more than 6 hours are probably goners.