Try turning off the phone and then turning it on in "Safe Mode". While most apps (especially ones you have installed yourself) will not run in this mode, you will be able to see if any apps auto launch or try to while in it. If not, then you probably have a bad app, virus, malware, etc., on the phone that you need to remove before it will work normally.
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.