Top of the list is to make sure your phone is getting power. You've tried different chargers and power cords, right? Wiggled the plug when it's inserted? Checked the charging port for dust, dirt or debris and cleaned it with 90%+ isopropyl alcohol? Examined it with a magnifying glass and a bright light looking for bent or missing pins?
So what was happening with the phone before it died? Has anything changed? Updates installed, phone dropped, got wet, run over by a snow plow, etc.? Has it been charging correctly up till now? Battery life normal?
Give us more detail on the history of the phone, because right now we don't have enough information to be able to help you.
At some point you're most likely going to have to open the phone, so you might want to order some tools and display adhesive so you're ready when the time comes.