Try this:
Type Services.msc in the run box.
In the list which follows, scroll down to Windows Defender, (you will, in your case, probably find it is disabled or on manual.) Whatever, double click it and select "automatic" if you want it to be permanent, or manual, if you want it for the current session. Now click "Start" .A message will probably pop up to the effect that it started and stopped again.
Ignore this and now close the window and, through the Control Panel (icon view) click the Windows defender and you should find it fully functional. Open the options and make sure you do have it in use (Somewhere near the bottom of the list - sorry, I have it disabled so cannot help there)