Issue

Windows 10 Security Centre shows red X next to Virus & thread protection. Status reads “The threat service has stopped. Restart it now.“

Clicking Restart Now button doesn’t help. Following error is displayed “Unexpected error. Sorry, we ran into a problem. Please try again.“

Resolution

I suspect the issue was caused by a third party antivirus disabling Windows Defender protection and then not re-enabling after it was uninstalled. To fix I had to re-enable Windows Defender via Registry:

• Open Registry Editor (regedit.exe) and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
• Change DisableAntiSpyware and DisableAntiVirus values from 1 to 0.
  
• Save changes and try restarting Virus & thread protection again.

October 2017
Windows 10 Pro (1709)