That looks like yet another broken library database.
Search the forums for 'empty library', there you can find tons of other threads with helpful infos.
Most of the times, the only solution is deleting the library database file, which are located under:
C:\Users\YourName\AppData\Roaming\Winamp\Plugins\ml
I highly recommence that you do periodic updates of your whole Winamp settings. I've written a tool which can do it for you, and restore the files when needed. Take a look at the 'Winamp Backup Tool' link in my signatur.
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