First, try force closing the music app, which you can find out how to do here:
Reopen it and see if they're there now - if that doesn't work, I would reset the device:
Press and hold the Sleep/Wake button and the Home button together for at least ten seconds, until the Apple logo appears.
And if that doesn't resolve the issue then I would backup the device to iTunes and restore it. Just be cautious and back up first, since restoring deletes all content and settings:
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