Can you manage to transfer the file to your computer? If so, when I've had problems like this in the past with files on my iRiver, I've used Audacity>File>Import>Raw Data and that's done the trick.
If you can't get if off the iRiver then that's a different problem.
Hope that helps.
As an aside I used to regularly get problems like this with my iRiver, but since I updated to a more recent build of Rockbox it hasn't happened once, so it may be worth updating if you haven't for a while.