I deleted the extension at that path, but all this seems to do is remove the extension. Then I have to run the repair installation to get it back (when then puts it back at that path).
I've verified that this happens whenever I open more than one instance of Visual Studio. For example after the above repair, everything was fine. The Typemock menu was back in Visual Studio, and I was able to run tests.
I then opened another instance of VS with a different solution, and this instance did not have a Typemock menu, and under Tools/Extension Manager, Typemock was disabled with the Enable option grayed out. When I closed and reopened the original VS instance, it was then in the same condition: no Typemock menu, and the extension disabled.
In other words it appears that opening more than one instance of VS disables the extension, and the only way to get it back that I know of is to rerun the Typemock installer in repair mode.