Have you tried starting the device in "Safe Mode"? It will start the device running only what should be on it, nothing that was installed later. It is so you can determine if there are any apps or software on the device that are causing your issues. Try the following and see if it allows the device to start up. If it does, then you have some searching to remove whatever is causing your problems.
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 safe mode, simply repeat the steps above, and you should see "Safe Mode: Off" rather than On.