What do you mean by won't start? Not turning on or blank screen?
You can try hard reset (turn off your device then press the power button along with the volume down button hold the volume down button until you see a big ! Mark then press the buttons in sequence (find the correct sequence before trying) volume up volume down power button volume up your phone will start resetting) or you can use device recovery tool.
Note: Both of the above will erase data from your phone.
Device recovery tool will take you to windows 8.1 which can upgraded to windows 10 later on