This explains how to fix missing photos in Lightroom
If you can't find a particular catalog, have your operating system search for files whose name ends with .LRCAT and then once found, double-click on it to open it.
By the way, this is very unclear
I thought I would start with my older work because I was worried about doing this. From what I do know, I believe that during the process, my Seagate external drive may have ejected itself. This caused an error and the process couldn't continue. I plugged it back in and repeated the step so the remaining files could be transferred over. When it was done, I started clicking on my images in LR and realized they all have exclamation marks and show as missing files.
because you mention a process, but you never describe what the process is. If you are copying files, then nothing in Lightroom should be missing. If you are moving files, then you would wind up with missing files. If you did something else, then who knows. It would be nice if you told us exactly what "the process" actually was.
Great idea about searching for .LRCAT! I did that, but I only found one catalog? I can't find my new one? I'm so perplexed...
Did you search all of your hard drives, external and internal?
Also, do you actually know right now where all of your photos are stored? Can you find them using Mac Finder? Do they all look to be present in the expected location?