I have experienced this twice. Today I tried the file recovery programs mentioned in another thread, which unfortunately didn't work for me.
What I did find, and this worked the first time as well (although at that time I didn't realize what was going on) was this.
With the phone connected to your pc browse to the storage card through the windows mobile device in my computer. Drag a file into the folder from your pc and see if the list re-populates. This has worked both times for me.


Good luck