Good afternoon laurafromwantage,
Welcome to the Apple Support Communities!
I understand you just installed iTunes on your laptop and tried to sync it with your iPhone. As you have discovered, the syncing process only goes one way (from the computer TO the iPhone).
If you have a backup of your iPhone, you should be able to restore from a backup and get your music back onto it.
Restore your iPhone, iPad, or iPod touch from a backup
... some 2000+ songs accumulated over many years through various different systems
It sounds like the 126 songs that you have are purchases from the iTunes Store. You would have initially imported all of your music into an iTunes library, so hopefully you have a copy of that iTunes Media folder on a computer, or backed up via Time Machine so that you can copy it to your current laptop and then sync again to your iPhone.
How to move your iTunes library to a new computer
If you are not able to restore from your iPhone backup, and you can't find the iTunes library that contains the music you have collected over the years, then one other option that you have is to subscribe to Apple Music. This service allows you to stream or download anything from the Apple Music catalog (40 million songs) for a monthly fee to any of your devices. There is a free three month trial if you want to check out this option. Click on the "Try it free" link in the upper-right corner.
Take care.
