Same problem with mine, and it applies to a few different music players. It seemed to happen suddenly too. I started a bug thread but it seems to have been widely ignored. I have even got a new SD card and the same problem applies.
I'm trying to do these:
Settings >> Applications >> Manage Applications >> All tab
For each of the following, I clicked on Clear data and Clear cache: (Just clearing Gallery did not do it for me)
Media Container Service
Then I restarted phone, but nothing, only shows me the wallpaper and lockscreen.