Presumably then, each song now has an exclamation mark to the left of the song's title.
The error message you've seen suggests that the song's files have been moved, renamed or deleted, or one of the folders in the path of the song has suffered that fate. While yo can do this in iTunes, if you do it using Windows File Manager, iTunes will no longer be able to find the file because the file is not where it was.
Have you, or another programme on your computer, done any of that? Alternatively, have you recently moved your Library, or done other "housekeeping"?