Lollipop

You may just have to factory reset you device to get it back up & running.
if you choose to go that route back up your data and pictures and contact with Samsung kids on your PC.
To preform a factory data reset power your phone off, hold down the power button volume up button and home button at the same time until you boot into recovery. Use the volume rocker to select factory data reset hit the power button to make it happen. let it do its thing then select reboot system and hit the power button.
this is usually a last resort if clearing the cache doesn't work. remember to make backups first because this will wipe everything