The Great PSN Crash of 2010, thankfully ended abruptly yesterday evening. However, with that collective sigh of relief comes problems surrounding unsynced trophies. If some of your trophies are missing, worry not, there is an easy fix.
Sony issued an official statement to CVG.
“It is possible that some users who experienced the connection problems yesterday and tried unsuccessfully to access a game (online or offline) may find that trophy dates for that title is no longer displayed.
“To resolve this issue, the user will need to re-access the game, reinstall the trophies, then exit the game and sync their trophy collection to the server in order to REGAIN previous achievements.”
[Source]
