Apparently something happens to the definitions in Windows Defender that stops updating from working.
Click to enlargeThe following command line procedure corrects it.
1. Type Windows-R (Hold the Windows or start key and type an R) to bring up the run dialog box.
2. Type in cmd.exe and the press ctrl-alt-enter to run it as Administrator.
It may ask for escalation privileges so type in your password and press enter if it does.
3. Use the cd command to change to the Windows Defender directory:
cd “\Program Files\Windows Defender”
4. Run the following commands:
mpcmdrun -RemoveDefinitions -All mpcmdrun -SignatureUpdateClick to enlargeThanks to
Tim Malloroy for this excellent tip.