Hello Dennen23.
Thanks for reaching out to Apple Support Communities. We understand that music on your iPhone synced from iTunes has disappeared. We will be glad to assist.
iTunes for Windows has not been discontinued. Computers using later versions of macOS no longer require iTunes. See this for more information: What happened to iTunes?
"If you have a PC, you can continue to use iTunes for Windows to manage your media library, make purchases, and manually sync and manage your iPhone, iPad, or iPod touch."
Please see the following resource for some help with getting music back on your iPhone:
If songs are missing from your music library after you turn on Sync Library
Cheers.