It's all about the battery...
If you've taken your iPhone on holiday with you, either to a sunny climate or a land of freezing temperatures, you may have noticed your battery acting up. Apple recommends that iPhone devices are kept between 0° and 35°c.
Taking an iPhone into extreme cold can cause the battery to quite literally freeze and become temporarily unusable. Extensive research has found that the life of a battery decreases the colder it gets.
The energy within a lithium ion battery flows through an electrolyte liquid. When that liquid gets cold, crystals form, preventing the flow of energy. If the liquid gets cold enough, it can freeze, causing an entire stoppage to the flow.
While this temporarily stops an iPhone from working, it will begin to function again as the battery warms up.