So your game data was still there but your game save and your trophies aren't showing up either? The trophies should be linked to your account more so than the game save itself.
There is an option to 'salvage' files within the GT5 options but I really don't think that will help in this case.
If the trophies aren't showing up either, it seems more as though your PS3 User Profile has corrupted or you accidentally logged in as another user.
Ah damn okay, hopefully someone knows of a way to restore game save files or try the GT5 in-game option to salvage files as a last ditch attempt.
Can't say that I've ever lost my game save but I back up to USB just about every day knowing the risks. Especially during updates.
Try not to stress too much about what you have lost progress-wise, even I have started the game over several times and achieved Platinum twice now.
If you struggle with any events or can't find the cars you need to complete events -- that's what we're all here to help with. 
PS: If you haven't yet achieved the Penniless trophy -- this is the ideal situation to get it.
Buy the 19,000Cr Mazda from the dealership, install the CPU engine upgrade for 1,000Cr and return to the home screen.
Get your trophy, quit the game, delete the game save (purposely this time) and start fresh with whatever car you prefer.
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