The best way will be to Restore the entire Library.
There are 2 ways to do it.
1) Open WMP and Right Click in the Empty space to the left of the Play Tab.
Choose Tools--> Advanced --> Restore Media Library
(As seen in the snip below) WMp will automatically close.
Re-Open Media Player and wait for it to rebuild the Library.
2) If the first method does not work for you, then try this method.
Close WMP & Make sure 'Show Hidden Files" is enabled.
Browse to the hidden Folder @
C \ Users \ your user name \ AppData \ Local \ Microsoft
Delete the folder "Media Player"
Open Media Player and wait for it to rebuild the Library.
Either way will completely remove everything in the library itself.
However, it does NOT delete the actual files. Only the reference to them.
Once you re-open WMP it will re-build it all.
Hope this helps.