Start over and over, by restart or start

[AC Question]

Because of a problem, I restart mine phone, the phone start up, but nothing happens. After a couple of restarts, I get the inlog screen, but before I can give mine pasword, restarting. Over and over. At home, I restart without problem.
I have changed a sim card, recently, maybe this is the problem. Proximus I don't trust them for a cent.
Or the phone is stuck. Or software fault.