This is a question that I posted, and then after much digging finally resolved myself. There is actually quite a bit on this subject both on this forum and elsewhere, but it usually requires some familiarity with Terminal. I am going to describe the problem I faced and then describe step-by-step in detail (at a beginner's level) how to resolve the issue in Terminal.
In short, I checked out a previously committed version of my app in Xcode, which - because it was a version from several weeks prior - did not contain my most recent commits. In other words, I had no access to any of the commits that contained my most recent work. They had all disappeared.
My commits and pushes had not stored in GitHub because presumably some time before this I had accidentally selected my main folder as the destination for my commits, rather than one of the two branches I created. So I had absolutely no access to my work. By all appearances I pretty much had to start from scratch.
After much hand wringing, teeth gnashing, and hours of scouring the webs, I finally uncovered the solution. But it takes a bit of understanding about Terminal to make it work. So after several more hours of learning Terminal, I finally successfully restored all of my work.
For any of you who are new to coding (as I am), and who have no experience with Terminal, I will provide detailed instructions on how to resolve this issue if you encounter it in the answer below.