First, have you tried pulling the power (battery/cord) and letting it turn fully off, then turning it on? If not, I would try that first as it would be the simplest and safest way. If that doesn't work you can try starting the phone in "Safe Mode" and see if it will let you in. If it does, then you probably have something (app, file, virus, malware, etc.) on the phone that is the cause of your problem and it will need to be removed. You would have the ability to access the files in "Safe Mode" and connect the phone to say a computer via USB cord to take the data you want off the phone (photos, music, contacts, etc.) and then if necessary do a "Hard Reset" of the device to wipe everything else and start fresh.
To get into "Safe Mode"...