Many people have experienced this problem today, myself included. The official solution offered by Take2Games might seem a bit frivolous to some, especially having to run through the list of some twelve steps.
Before you go ahead and say goodbye to your hard work in deleting your saved game (as suggested by T2), try signing out of the PlayStation Network (PSN). This can be done by either canceling the sign in process before it can be completed under the PlayStation Network option on the XMB, or - once signed in - navigating to your own PSN ID under Friends, hitting triangle ( ) and opting to sign out. For a permanent fix (i.e. so you don't have to go through this every time you turn on the system), disable your internet connection via Settings -> Network Settings -> Internet Connection.
Note that if you simply sign out of the PSN - rather than disabling your internet connection - you may still access multiplayer without having to quit the game and re-enable your connection. After selecting Multpilayer via the mobile phone, you'll be prompted to sign back in to the PSN. If successful, you're free to enjoy the additional online experience Grand Theft Auto IV has to offer.
• T2's official solution
• General freezing discussion
• Signing out of PSN
• Deleting the game data utility file
Please confine discussion on this problem to the existing topics. Thank you.