All my music on iTunes was deleted but is still on my iPhone how do I get it back onto iTunes?

Apple's music syncing tools, like iTunes is a one way sync from most PC/Mac to a device.

There may be a 3rd party solution, so an internet search might get you a solution.

Restore your computer's backup to retrieve the music that was not purchased in the iTunes Store.

For the music that was purchased in the iTunes Store:

Redownload apps, music, movies, TV shows, and books from the App Store, iTunes Store, and Apple Books - Apple Support

Re-read the two responses above.