I started with a blank HTML5 page and had some code which did work.
Suddenly I do not see the other tabs in the Intel XDK. Looks like some problem with the file.
What might have caused this?
You've created an IoT project, not a mobile app project. You probably started with an IoT template or sample. You'll need to start a new mobile project and copy your sources into that project.
Intel XDK used: 3491
Windows 10 professional
That process is definitely correct. However, when I look at your initial image > https://software.intel.com/sites/default/files/managed/8f/60/AnotherProblem.PNG < it looks like you have an IoT template into which you've inserted an index.html file. That main.js file is typical of an IoT blank template and there is no reference to a main.js inside your index.html file. Which leads me to the belief that an IoT blank template was used. Note that in 3491 the IoT samples and templates are presented first, the mobile app templates and samples are in the lower half of the "start a new project" page.