Summary

It's possible for Windows update and notably Defender signature updates to become corrupted.

Sometimes the below can fix it.

Defender

Refer https://www.microsoft.com/en-us/wdsi/defenderupdates

Clear and force Update:

cd %ProgramFiles%\Windows Defender
MpCmdRun.exe -removedefinitions -dynamicsignatures
MpCmdRun.exe -SignatureUpdate

Win Updates

Fix Device Manager

chkdsk /f C:
DISM.exe /Online /Cleanup-image /Restorehealth
sfc /scannow