PlayStation Network ID
Trophies are stored on your PlayStation 3 according to your PlayStation Network ID. If you have multiple usernames on your PS3, you may be logged in under the wrong username. You may change your user name under the Users option in the Xross Media Bar.
Automatic Login
The primary account on your PS3 will log on automatically at startup. If your account isn't assigned to log on automatically, your trophies won't load when you turn on your console. You may control the automatic login options by scrolling to your username in the Users menu on the XMB and pressing the triangle button while highlighting your username.
PlayStation Network Outages
The PlayStation Network has experienced several outages since the PS3 was released at retail. PSN outages have lasted from several hours to several weeks and have ranged in causes from internal errors to malicious hacking attempts. One such internal error occurred on March 1, 2011 when the internal clock of some PS3 tried to adjust the time as if it was a leap year and prevented these consoles from connecting to the stored information on the PlayStation Network.
Modified PlayStation Consoles
Modifying your PS3 by jailbreaking the PS3 operating system or installing a mod-chip to the console's hardware may limit your access to the PlayStation Network. While modifying your console isn't illegal, Sony has outlined in its terms of service and warranty documents that modified consoles won't be serviceable under the PS3 warranty and may be denied access to the PlayStation Network. Access to the PSN is required to access the trophy information stored on your console.