missing songs on Itunes latest version

I have a little over 6000 songs on Itunes 12.3.3.17. Yesterday I noticed many (over 400) were missing or could not be located. The little exclamation point to the left of the song indicated this. So I spent considerable amount of time finding the songs online and replacing them. I added maybe 20 songs. Today they are all back. Nothing has changed in my library except the 20 or so songs I added. Is there an explanation for this? Is this a corrupt program?

Thanks
Mark

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