Why is there a padlock with the word custom?

[AC Question]

I have an att phone and switched to cricket. I turn on my phone its a samsung galaxy note 3, I just recently switched my motherboard put. But now I have a padlock with custom word on the screen and then it goes to the att screen it freezes and then sometimes it goes through but then it shuts completely back off..what do I need to do to get it working again?