My iPhone won't turn on

Start by connecting your iPhone to a charger. If the battery is completely discharged, it may take some time before the phone turns on again. Also try a different charger or cable.

If your iPhone does not turn on, you must visit an authorized Apple repeater. Start here:

Apple Repair and Repair Status Check - Official Apple Support

Try following the steps in this Apple article:

If your iPhone won't turn on or is frozen - Apple Support