Connect your phone to the pc, delete the music, reboot it and see if it can open again. If so reload the music.
If it still won't open you will need to reset your phone. Make sure you back up everyting that you want to keep before you try it.
Hard reset:
1. Switch off the phone.
2. Hold down the following three buttons: Green (the call answer button), * button, and '3' button.
3. While holding these buttons, press the power button and switch on the phone.
4. Release when the phone shows the nokia logo or shows other signs of life like the language selction screen.