Hmmm... I know you did it once, but try running the Scan Disk command again (Settings menu). If that doesn't do it, do a proper power down sequence, wait 30 seconds and then power up again.
Proper Power Down Sequence: Press and hold the STOP button until the word SAVING appears in the display, wait until the display goes back to the clock, then turn off the power. Whenever you want to power down, do it that way.