iPhone temperature error and shutoff

In the past few days my iphone has been overheating, and then showing the error message "Temperate- iPhone needs to cool down before it can be used" repeatedly. Today it did that and turned off and won't turn on again or charge. I tried holding both buttons for 10 seconds and nothing happened. It hasn't been in any hot environments or anything and I keep apps closed most of the time.

Why is it doing this and what do I do to get it back on/prevent it from happening again?