Hi there, JayneyW.
It appears that you're missing music that you've imported to iTunes from CDs you've purchased. Some are downloading from iCloud to your Windows PC, however, and we're glad to help with this.
If songs are missing from your music library after you turn on Sync Library -- If your music is downloading from iCloud, then you're either subscribed to Apple Music or iTunes Match. This article provides information and steps to determine why your songs aren't downloading.
Identify cloud status icons in your music library on your Mac or PC -- You can see what each iCloud status icon means with this article.
Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC -- If you backed up your iTunes Library before joining Apple Music or iTunes Match, your songs should still be available there if you can't download them from iCloud.
Take care.