If it let you into "Safe Mode" then there must be something on the device that is interfering with it working correctly. Bad app, virus, malware, etc. I am including the "Safe Mode" instructions below (just in case). However, should it not let you in, then you may well have to "Hard Reset" the phone.
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.
NOTE...