Q: Phone Unresponsive

Recently my iPhone 6 started restarting all on its own without me (turning off and on). Then I cracked my phone and it only had one crack across the middle of the screen and it didn't seem to be a damaging crack (compared to other cracks before. Then for the past few days my phone has been acting normally except that it continued to turn off and on without me. Finally this morning I woke up, after my phone had been charging all night, and the screen was on but black and my phone did not respond to me at all. What do I do? Is there any way I could get it fixed without having to buy a new phone? I don't have insurance for it and don't want to buy another one for $600.