I would recommend reading this article: If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support
After you do those steps (and if they work), you could just erase the iPhone using iTunes: Restore your iPhone, iPad, or iPod to factory settings - Apple Support
You don't HAVE to erase your iPhone using iTunes—you could just redo what you did before—but it is an alternative method that I think you should consider.
Worst case scenario, if the iPhone remains unresponsive, contact Apple Support: Contact Apple for support and service - Apple Support