I know that this was posted awhile ago, but this does happen occasionally. For some reason the game just sort of goes into limbo and doesn't finish what it's supposed to. Sometimes it's loading the track, sometimes, like in this case, it's a race.
The cars will continue to go around the track endlessly, they tend to drive in a "rolling start" manner, not full speed. Basically, the AI knows that the race is over, but the final lap was not counted as completed according to the game.
The only thing that I'm aware that you can do in this situation is to hit the PS button and choose, "Quit Game". When you start the game back up, go into the Options, and scroll to the bottom. Choose the one that says, "Network" and clear the System Cache.
I don't know if clearing it will actually help, but it surely can't hurt. The good news is that this rarely happens and it's not likely to happen to you again for quite some time, if at all.