1. If the battery is completely empty, charge the phone for at least 20 minutes using phone's wall charger. Wall charger supplies 1A current, if you try this with computer USB, those typically supply less current and charging time is much longer. Then try to switch on the phone again.
2. If your phone powers on, try the charger with another compatible phone to check if the problem is with the charger.
3. Reset your phone:
- Phones with removable battery: Simply remove the battery for about 30 seconds.
- Lumia phones with integrated battery: Leave the phone connected to charger, then press and hold the power and volume down keys simultaneously until the phone vibrates. Release and the phone will reset.
Note: After reset, you may need to set the date and time on the phone.