Lost Collections and thousands of images???????

Help!!!

I had to uninstall and then reinstall Lightroom Clasic CC.  Then I had to upgrade.  After upgrade, I have lost many, many collections and thousands of images.

I have been syncing with my mobile device and the collections all appear there, having once been synced.

Where on my PC desktop can I find my old collections and how do I install them into the upgraded version?

Martha in Maine