Try starting it up in "Safe Mode" and see if it will load. If it does, then you likely have something on it you need to remove. Bad app, virus, malware, etc., to get it to work correctly.
However, if it won't even start up in "Safe Mode", then you will either need to do a "Hard Reset" (which will wipe the device and start it as it was when it was purchased, your stuff gone) or have it looked at by a tech (who may reset it anyway).
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...