So there I was testing a simple web page, adjusting this & that. Suddenly, when I tried to recompile it I got this message:
This control resides in the header on the Master Page of my project. There's nothing fancy or special about it. I did a comprehensive search and found lots of others who had encountered similar problems with controls suddenly not being recognized. I tried all of their solutions but to no avail.
So eventually I copied main.Master and main.Master.cs outside of the project folder and deleted these two files within the project. Then I created a new, blank version of them. Slowly, I started copying the markup code and C# code back into their respective files, being sure to perform a "Rebuild Website" every so often. Lo and behold, when every last line of code was back, it worked! Something internally must have changed but nothing in my code files did. Another 2 hours wasted. :-(