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Check to see where they're at using a file explorer [storage> internal/external card>android>data>com.zubersoft.mobilesheetspro>files]. If they're all there you may be able to 'select all' and copy them to a folder somewhere and re-import [or sideload] them. You will probably need to watch out that you don't bury them too deeply into another folder [pdf in a folder that's in a folder]. If this happens look at settings>storage>create sub directories for each song [mspro], I think un-checking this would fix that problem. At the minimum you could do something like this for what you need now.
There's probably a better way that I don't know.

I just happened to think, if you have an external sd card maybe it's looking for the file on the wrong sd card.