Imported downloaded tracks disappearing

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In this past week either some or all of my imported music has magically disappeared from my phone. The first time this happened I noticed that the download playlist button wasn't toggled on half of my playlists of imported tracks. Today I've discovered that every single file I had downloaded is no longer available. All of the buttons are set to download but every last song is greyed out and unavailable. This is extremely irritating and inconvenient. Help is greatly appreciated.