Yesterday while using Rockbox in the car on a long trip we had set it to shuffle through a directory of about 300 mp3's. I turned it off at a gas station, but when we were back on the freeway I noticed it had completely frozen at the end of one of the songs.
I tried resetting it innumerable times w/ <menu><select> combo, but it would just go back to a frozen play screen and no audio output after reboots. Even tried booting into the Apple firmware and back. When we were out of the car, I booted into the Apple firmware (the only way the machine would do anything), hooked up via USB to the laptop, and went into the .rockbox directory. There I found the .playlist_control file and deleted it. After reboot into Rockbox, it complains about missing the Playlist Control File, but creates it and works normally again.
FIX: if you are stuck in a "freeze-loop" and reboots don't help, go into USB mode and delete the .playlist_control file from .rockbox directory. It will be recreated when you reboot and your problem will be solved.
Maybe someone will want to add this to the wiki, or is this old news?