As you may have read elsewhere, yeast is a living organism and reacts to different temperatures in different ways.
If it gets to cold it will go to sleep and may give you the false impression that its finished fermenting. If it gets too hot it can actually kill itself completely or partially. That will be determined by how hot it gets.
To hot? Along the road to getting to hot your yeast will also create some unpleasant tastes and odours that, while not undrinkable, they wont be enjoyable. And… we all want to enjoy our favourite beverage that we created. As the temperature rises you create a chain reaction and the more it warms up the more active it becomes and therefore creates even more heat and will do more damage to you beer, wash or wine.
Too Cold? As I mentioned briefly above, your yeast will become totally inactive when it’s too cold. The major concern with this situation is that when it warms up and becomes active again this can cause all sorts of problems if your product has been bottled. The activity creates an excessive amount of gas that when trapped in the bottle will cause the bottle to explode. If its a spirit wash that hasn’t finished you will not get your expected result when you distill it.
Your hydrometer is the only thing that will give you an accurate reading and should always be used when testing any fermented product.
For me information see ‘Why is my beer exploding?’