Windows 10 Antivirus Problem...

First way;

Please run "regedit" as administrator

in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender

and poof! there is it I can run my windows defender again!. There is 1 value when I saw it then when I turn it to 0 its okay again!.

Reboot...

if windows defender don't start

Second way;

Please run CMD as administrator and type these commands one by one: 

Dism /Online /Cleanup-Image /RestoreHealth
SFC /scannow

Source