I was #6 on the score board for puzzle 500 last night. I left my computer running a script overnight. This morning it took a minute or so for foldit to paint the screen when I opened the window, and I was at #2 (9450). I assumed there was a memory leak or something and it was time to restart the client, which I did, saving the work-in-progress (with a score of -30000 or so). When I restarted it I had disappeared from the score board, and when I did a wiggle I reappeared on the score board way down at the bottom.
Try restore very best. Maybe you lost connection and your client was not sent your best solution to server.
Yes, Restore Very Best worked. Unfortunately I had spent some hours working on a new fold of 500 before I discovered that :-)