I had the exact same problem with my girlfriends sony nwd-b105 mp3 player. I fixed it by downloading the firmware version 1.2, which can be found here at the sony support link below.
- Backup your mp3 files
- Make sure your mp3 device is NOT plugged in
- Run the firmware executable
- When it asks for it plug in the mp3 device
- Check option: format data area
- Press Start
- When finished, take the mp3 device out and press the power button
- If everything went well it is creating a database again and will function correctly as usual
- Copy your mp3 files back to the mp3 device
- Enjoy your music again!