I had this problem before with assasins creed brotherhood. If this is the same thing it is because you may have tried to play the game without a disc in the drive. When you exit the game the xmb tries to bring up the disc icon for some reason then crashes and locks the ps3 in a startup/lockup cycle. If you enter the recovery menu, restore the ps3 from there. If I remember rightly though, you may have to format the hdd to cure this.