Well for heat, it could cause the crops of farmers to over heat and break down, along with destroying the food supplies. (over heat will cause dehydration quite fast and quickly!)
The cold would break the food and make it much harder for people to survive without it. They could end up with frost bites, if not treated immediately. (that'll kill the human body if they're frozen cold, if it reaches the heart, human body will immediately die)
If this happened human kind (along with animals) will die eventually. The city life will end up I terrible condition (along with regions around the world)
I hope this helped! I tried my best to explain as much as possible!!
