Try starting it up in "Safe Mode" and see if it will load without flashing. If it will, then you have something on the phone that is the cause and will need to be removed. Bad app, virus, malware, etc. If, however, it still does the flashing in "Safe Mode" then it is likely a hardware problem and you should take it to a local tech to have it repaired.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.