Step 1: Enable Windows defender from control panel
1.Press Windows + X, Click on control panel.
2.On right top corner click on View and then choose large items.
3.Now from the list click on Windows Defender and try to enable it.
If not try to enable it from services.
Step 2: Enable Windows defender from services.msc.
1.Press Windows + R, to open run prompt.
2.Type services.msc and press enter to open services list.
3.Under services look from Windows defender service and start the service.