how to find Windows Defender in my computer?

Open Windows Security from the Start menu, then turn Real-time protection back on under Virus & threat protection.

Reactivate Microsoft Defender Antivirus

1. Select Start and type Windows Security.
2. Open Windows Security.
3. Select Virus & threat protection.
4. Under Virus & threat protection settings, select Manage settings.
5. Set Real-time protection to On.
6. For fuller protection, also turn on:
   • Cloud-delivered protection
   • Automatic sample submission

On Windows 10, the longer navigation path is Start → Settings → Update & Security → Windows Security → Virus & threat protection. Microsoft Defender Antivirus is built into Windows 10, so it normally does not need to be downloaded or reinstalled.^1^

Confirm that Defender is active

In Windows Security:

1. Open Virus & threat protection.
2. Under Who’s protecting me?, select Manage providers.
3. Confirm that Microsoft Defender Antivirus is shown as active.^2^

If the switch is unavailable or Defender stays disabled

Check whether another antivirus product is installed and active. Windows automatically disables Microsoft Defender Antivirus when a compatible third-party antivirus is running. If that product is no longer wanted, uninstall it through Settings → Apps → Apps & features, restart the computer, and Defender should reactivate automatically.^3^

If Windows Security, Virus & threat protection, or the activation controls are completely missing, that is a different problem from simply switching Defender off and may require Windows repair or malware-removal troubleshooting.

References

1. Antivirus and antimalware software: FAQ | Microsoft Support
2. Scan an item with Windows Security | Microsoft Support
3. Windows Security App Overview | Microsoft Support