My iPhone doesn't turn on

I set my iPhone 6 down on the table one day and it never turned on again. Hard reset doesn’t work, and I took it to the Apple store and they ran through all their tests, but the phone wouldn’t be recognized. Before, I had been having problems with dropping it slightly (like from a height of two inches onto a table) and the display turning off— the phone would still vibrate or continue playing music, but the screen would be black. Then, a hard reset seemed to make it good as knew, until the next time. I’ve already gotten a new phone, but I’m hoping to use the old one as a learning experience and fix it myself. Problem is, I have no idea what replacement might help— suggestions are very welcome!