Try starting up in "Safe Mode" and see if it will let you in. If it will, then you can save your files and you should also consider checking the device for viruses, malware, bad apps, etc. If it won't start up even in "Safe Mode" then a "Hard Reset" may be needed. Or worse, it could be hardware.
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.