As long as your knife blade does not exceed the temperature at which it was initially tempered when it was heat treated, it will not lose its hardness. This requires knowing the original heat treating temperatures, which is of course not practical. Striper is pretty much right on this one. For most blade steels, this temperature would probably be around 400F or higher, and you would probably have to get the blade up to well over that, maybe 500 or higher, for a measurable time, to do any noticeable damage.
The softening which occurs during tempering depends on BOTH the temperature to which you heat the steel, AND the time at which it is held at temperature. That means if you get it "too" hot, but only momentarily (like sterilizing it) you will probably not do any damage. Also, several of the higher alloy steels may be tempered at 1000F or even higher, so theoretically, you could heat them to 800 or 900, and still see not change in their properties.
The heat exposure will almost always discolor the knife, as striper also hinted. This is a cosmetic change to the steel surface, and does not necessarily mean the properties have changed in any way.
From a real-world view, it is unlikely that you would do anything accidentally or in normal use that would even come close to damaging the temper on teh blade, unless you were doing something that involved leaving the blade in a bed of coals or something similar.
As for cold, cold will do nothing to permanently change the temper of most blades. However, when heat treated steel gets cold, it gets more brittle while it is at the low temperature. Usually a relatively small change, like from 70F to -10F, would not change the properties TOO much. In a few cases, certain steels might be significantly less tough, but in all cases, they would return to their original properties when they returned to their original temperature (room temp, for instance.)
The one exception would be that certain steels (D2 is one) could theoretically undergo certain minor but potentially measurable physical changes if exposed to very low temperatures, depending on their original heat treatment. This is the principle behind the refrigeration or freeezing or "cryo" treatments you may have heard of. Once again, in the real world, it isn't likely that you would ever encounter such circumstances unless you were a polar explorer or used your knife in liquid nitrogen.
