My guess is that the new download placed the music in a folder of a folder or something along those lines. Now your player can't find it because it is looking one or two folders deep and the music is deeper than that. If you have a file manager app you can use it to search for your music and see where it is exactly. It used to be that it had to be in a folder on the root of your SD, so one folder deep, but I've noticed that a lot of my music has been placed in folders in that folder. So you could have a location that looks like this; SD>Music>Dave Mathews Band> Name of song. What it probably shouldn't look like is; SD>Music>Music>Dave Mathews Band>Name of song. You could test this by placing a song in the music folder and seeing if your player can find it; SD>Music>Name of song.
On another note, it used to be that the music had to be in a folder named music, but again I haven't noticed that being the case in some time however there are some folders that have .txt files in them that tell the music player to ignore them as sources of audio, you can do this for photos and videos as well, just saying. You may want to check the location for something like that which could cause the player to ignore the folder.
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