Oops. Thanks DR M. I saw that he said he was not "using" any 3rd party software. I meant what I said when I asked if any was "installed". There is a difference but I should have been more clear. My bad.
When 3rd party security software that has a real-time component is installed, the standard procedure is for that 3rd party software to "register" itself in the Windows Security Center. This tells Defender another solution is protecting the computer and Defender then will gracefully step aside.
IF, however, the user then disables the real-time component, Defender "may" still remain disabled. Typically, you can manually restart it, but that does not always work. So the solution is to uninstall the 3rd party solution.
Alternatively, [hopefully] the 3rd party solution will have a menu option to disable registering itself in Windows Security Center. Then both will [again, hopefully] work together without conflicts or excessive resource hogging. Malwarebytes Premium is a good example of this. In Malwarebytes Premium under Settings > Security, you can uncheck the option to "Always register Malwarebytes in the Windows Security Center". This will allow users to have both Defender and Malwarebytes Premium running at the same time. Fortunately, this is one of few exceptions and you can run these two security programs together with no problems.
Note this does not apply to the free version of Malwarebytes as it does not include a real-time component.
