Why can't I turn on Microsoft Defender?

Why can't I turn on Microsoft Defender? Any help on where to look?

If MBAM (MalwareBytes AM?) turns it off automatically, and if I've had both MBAM and Defender for as long as I've had this system, why now is Windows giving me an alert saying something like "Defender is turned off".

I left MBAM on and turned on periodic scanning. Hopefully that leaves me protected and without any concerning alerts. Thanks for your help.

Brink said:

You could also turn off registering MBAM with Windows Security to use both MBAM and Microsoft Defender at the same time.

It's what I do to have real-time protection from both.

If it's not "registered" then will it still work? Did it suddenly register itself (or maybe entice me to click something I didn't understand would register it) and that's why I'm getting the alerts about Defender not working?

Brink said:

You may have turned it on by mistake. You can turn it off in MBAM settings like below.

This will allow both MBAM and Microsoft Defender with real-time protection.

OK. I did that. But now everything looks different in Windows Security App and I can't figure out how to turn on Defender.

I got a page where I guess I'm supposed to Quick Scan or Full Scan. I tried Full Scan, but nothing really happened. After a bit it kicked me out of that page. There was a brief message which said "The administrator has turned off some thing or other". Then it kicked me pack to the front page where it says "Thread service has stopped". I can't restart it.

Brink said:

Close and reopen Windows Security to make sure it's fully refreshed.

Check to make sure "Virus & threat protection" doesn't have any notices.

If so, click on "Virus & threat protection", "Manage settings", and turn on everything.

I restarted Windows, including closing Windows Security. I'm not sure where to check if "Virus & threat protection" has any notices (because I can never find anything). This first pic is what happened when I tried to click the "Shield with a red 'x' on it".

Then I saw there was a system alert which said something about protection (I don't remember). When I clicked it, Windows tried to open Security at a Glance, but couldn't because it said "Page not available" (second pic).