Maybe the library is located somewhere else. Try holding down the option key when you start Music and see if you can find it.
Switch libraries with the Apple Music app in macOS - Apple Support
What macOS did you have before? If it was Mojave or earlier it might be in an "iTunes" folder instead of a "Music" folder. You can also try:
Music > Preferences > Files
and see where it is expecting to find you Music Media location to get an idea where to look.
Syncing is meant to go from the computer to the iPhone, not the other way around. Trying to import from the iPhone is not very fruitful, but if worse comes to worse you can try:
Recover your iTunes library from your iPo… - Apple Community