I just answered something similar on another thread. You can go through this.
You can try to soft reset first.
Volume down and power button until the phone vibrates. Phone will restart if it works.
Otherwise -
This is from the support site -
The problem you are experiencing may be caused by a software problem on your phone. You will need to perform a factory reset to try and resolve the problem.
Please note: All information on your phone will be deleted and anything that has not been backed up will be lost during this process. If you cannot access your phone's menu, unfortunately you will not be able to back-up any data.
Make sure your phone is powered off.
Press and hold the 'volume down' button.
Press the 'power' button until the phone vibrates, then release the 'power' button.
Keep the 'volume down' button held until you see an '!' on the screen, then release the 'volume down' button.
When the '!' appears, press the following buttons in order:
'volume up'
'volume down'
'power'
'volume down'
You should see rotating gears on your screen for around 5 minutes.
Your display will then turn blank for 30 seconds while your phone restarts.
It will take several minutes for your phone to restart. Once your phone has restarted you will need to follow the on-screen instructions and complete the initial set-up process on your phone.