Try this in order:
- Restart the PC.
- A stopped Windows Defender Antivirus or Endpoint Protection service can sometimes recover after a restart.
- Check that the Windows Defender Antivirus service is running.
- Open Start and run services.msc.
- Select Windows Defender Antivirus Service.
- Set Startup type to Automatic.
- On the Action menu, select Start.
- If Start is unavailable, select Stop, wait for it to stop, then select Start again.
- Make sure no third-party antivirus is disabling Microsoft Defender Antivirus.
- If a non-Microsoft antivirus is installed and uses Windows Security Center, it can disable Microsoft Defender Antivirus, which also prevents updates.
- Open Start > Settings > Apps.
- Find any unwanted antivirus or antispyware app and select Uninstall.
- Restart after uninstalling.
- Caution: Removing security software can leave the PC temporarily unprotected. Make sure Microsoft Defender Antivirus and Windows Firewall are enabled afterward.
- Verify internet connectivity.
- Open Start and run ncpa.cpl.
- Open the active connection and check that IPv4 connectivity or IPv6 connectivity shows Internet.
- If not connected, select the connection and use Diagnose this connection.
- Run the Windows Security troubleshooter.
- Download and run the troubleshooter from this page: Download
- It checks and can enable security features such as UAC, DEP, Windows Firewall, and antivirus protection status and updates.
- If security intelligence is not updating, confirm Microsoft Defender Antivirus is the primary antivirus.
- If it is, try a manual security intelligence update from https://www.microsoft.com/wdsi/defenderupdates.
- If Windows Update is also failing, run the Windows Update troubleshooter.
- Go to Start > Settings > Update & Security.
- Select Troubleshoot > Additional troubleshooters.
- Select Windows Update > Run the troubleshooter.
- If updates still do not arrive, wait up to 24 hours after confirming internet access.
- Some devices receive antivirus-related updates later, especially if they were offline.
- Troubleshoot Windows Defender or Endpoint Protection client
- Protect my PC from viruses
- Automatically fix Windows security issues
- Troubleshoot Microsoft Defender Antivirus Security intelligence not getting updated
- Troubleshoot problems updating Windows
- Important: Windows security updates and antivirus software