First of all Switch OFF your phone by holding the POWER button.
Now hold down, Volume UP + Volume Down + Power button together for few seconds.
Once you see the Lenovo logo, release all the buttons.
You will now see the Recovery Mode appears on your screen.
Now with the help of the Volume Down button select the option "wipe data/factory reset" and press the Power button to confirm.
Select "Yes delete all user data".
Use the Volume Down button to scroll and press the Power button to select.
Wait for the process to get complete.
Once completed, restart the phone by selecting the option "Reboot system now"
Congratulations! You have successfully formatted your Lenovo S650.
If that also doesn't work then try Android Repair software as it will help you to fix the Android System issues and thereby fix the issue.