Open the SIM card flap and find a small yellow button below it, use a non-sharp tool to press that button and leave the phone on a charger for 2-3 hours. Now try to start it.
If it doesn't start, you may need to try performing a software repair using Xperia Companion while your phone is switched off, if your PC doesn't detect it after following the guide, you might be facing a hardware issue and should contact your Local support to get your phone inspected.
