One should always be careful about what apps you download and install on a phone or tablet. Even through Google Play Store you can wind up with some real problems. Sadly.
Try letting the phone battery totally down. Then charge it but don't turn it on. When you do finally go to turn it on, do so in "Safe Mode". If the problem is a bad app, virus, malware, etc., you should be able to start up in "Safe Mode", then you can remove the problem and restart the device as you normally would. If that doesn't help, and it still won't start up in "Safe Mode", you may have to "Hard Reset" the device. However I would save that for a last resort as it will wipe all your personal data.
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.