iMac photos lost after plugging my iphone

I think I messed up big time.

My wife got me an iMac, so I started transferring everything to the new iMac. I got my external hard drive where I had old photo libraries and transfered all the photos to the iMac Photo app. Roughly 3000 photos. These photos were not in my iPhone or anywhere else for that matter. After I transferred all my external hard drive photos, I decided to try transfer my iPhone photos to Photo app. So I plugged my phone, it asked me if I "trusted" my phone, I click yes and the iMac and iPhone synced.

The problem it that when the sync took place all my iMac photos disappeared and now both my iMac and iPhone have only the photos that were in my iPhone, if that makes sense.

Is there a way to somehow recover the 3000 photos I had in my iMac before I messed up and plugged my phone?

Thank you in advance