You could try starting it up in "Safe Mode" and see if it loads. If it does, then you need to check the phone for bad apps, viruses and malware. Remove the problem and then restart the phone normally. If that doesn't work, then you are looking at either "Hard Resetting" the phone, which will wipe everything you have on it, or having it looked at by a tech.
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.