For some reason, could be malware … could be some online antivirus…. a new reg-key is added to your registry , just delete it and the windows defender will come back to live again :
REG DELETE "HKLM\SOFTWARE\Policies\Microsoft\Windows Defender" /v DisableAntiSpyware
You can also just change the value to zero.
it is very recomanded that you scan your system for malware, in this page there is free tools including microsoft scanner :