Check for Other Antivirus:

Defender automatically disables when third-party antivirus is installed. This is normal behavior. Choose one or the other.

Enable from Settings:

Settings > Privacy & Security > Windows Security > Virus protection. Turn on Real-time protection.

Check Group Policy:

Type gpedit.msc in Run. Computer Configuration > Administrative Templates > Windows Defender. Ensure "Turn off Windows Defender" is Not Configured.

Check Registry:

HKEY_LOCAL_MACHINE > SOFTWARE > Policies > Microsoft > Windows Defender. Delete DisableAntiSpyware key if present.

Run SFC and DISM:

Command Prompt as Administrator: "sfc /scannow" then "DISM /Online /Cleanup-Image /RestoreHealth". Repairs corrupted system files.

Windows Update:

Defender updates come via Windows Update. Install all pending updates.

Conclusion:

Third-party antivirus is the most common cause. Group Policy and Registry for advanced cases.