What output are you creating? WebHelp, CHM?
If WebHelp, did you copy the output to the mergedProject folder of the main project output?
Actually, I solved the problem by doing what I thought was counter-intuitive: I removed the several topics that I'd created, compiled and -- voila -- it's working again ... topic compiles, with form allowing me to look at finished project, and reappears in the Main Project... however, a further problem remains, that of non-functional Index and Search tabs. I'll start a new discussion for that one.