If you already checked to make sure it's not the battery, nothing we can do online, you need to either get a phone repair place to check it and fix it, or replace the phone.
Did you try removing the battery, holding in the power for 10 seconds, then replacing the battery? A few people tried holding in the power button for a while, releasing it, and holding it in again and their devices turned on after about 10-20 seconds of doing that.