"Hi, there. I cannot open Windows Defender in a Windows 8 computer, so please tell me how to solve the problem ASAP. Always appreciate your kindly help."
The reason resulting in the above-described problem is probably that the Windows Defender is disabled. And according to different methods of disabling the app, there are corresponding solutions which are clearly illustrated below.
If the system prompts you that the program is disabled by Group Policy (see the following picture), you can follow the steps below to have it enabled.
Step 1: Open Local Group Policy Editor.
Step 2: Locate and open the "Turn off Windows Defender" setting.
FYI, it is in Computer Configuration/Administrative Templates/Windows Components/Windows Defender.
Step 3: Choose Not Configured (or Disabled), and click OK.
Suppose you are informed that Windows Defender has been turned off and you can use Action Center to check its status (as shown in the following picture), you can enable it in the Action Center, referring to the procedures below.
Step 1: Access Control Panel, input action center in the top-right search box and tap Action Center to get into it.
Step 2: Tap the Turn on now button on the right of "Spyware and unwanted software protection (Important)".
If a prompt message tells you that Windows Defender isn't monitoring your PC because the program's service stopped (as exhibited in the picture below), you can go to enable the app in Windows Defender Service with the help of the following steps.
Step 1: Open the Search bar by Windows+F, type services in the empty box, choose Settings and tap View local services to open Services.
Step 2: Find and double-click Windows Defender Service.
Step 3: As the Windows Defender Service Properties window turns up, in General settings, click the bar beside Startup type, choose Automatic from the drop-down list and tap OK to complete the setting.