This is old news, if you change memory cards in slot 1 and save the game, the garage empties out except for the car you're in at the time. In this case, He'd just won a prize car and apparently that stayed, too.
The game save is actually 2 databases, the garage and the game status. You may have noticed that if you change cars, quit the game without saving, the next time you start the car is still the one you changed to. When you change cars you see the memory write icon. The garage database is being updated. When you save the game, the game database is being updated. If you've changed cards, there is no garage database, and you lose all your cars.
The only way to back up the game, i.e. have two full saves, is in the browser. If you've copied the GT4 save to another memory card in the browser, then you have a copy you can fall back on. If you've changed cards and saved, the original card MIGHT be OK, but don't try it without turning off the PS2 first.
There's no SAVE GAME and LOAD GAME process like there was in earlier GT games, this one (and Tourist Trophy, BTW) is different.
This is NOT the GT2 garage bug. Technically, it's not even a bug, it's by design. I haven't started a GT4 game in quite a while, but I believe you are warned onscreen during the initial setup not to change the memory card during gameplay.