Hi Robynn,
Thank you for visiting Microsoft Community. Third party antivirus applications may conflict and stop the Defender and it could also be caused because of virus infection.
Perform a full scan using Microsoft Safety Scanner insafe mode with networking and boot to the normal mode and check if Defender starts.
Microsoft Safety Scanner
http://www.microsoft.com/security/scanner/en-us/default.aspx
Note: Any data files that are infected may only be cleaned by deleting the file entirely, which means there is a potential for data loss.
If the issue still persists, try to perform a system restore to earlier date (probably 3 days) check if that helps.
- Press ‘Windows Key + R key’
- In the run window type ‘rstrui.exe’ and click on
OK
- Click on next in the System Restore window.
- Now choose the restore point of your choice, and try to restore your computer back to earlier state.
System Restore disclaimer: When you use System Restore to restore the computer to a previous state, programs and updates that you installed are removed.
If the system restore gets the Windows Defender back in the Control panel, try to start it. If Windows Defender fails to start, use the following McAfee Consumer Product Removal tool (MCPR), try to remove McAfee from your computer.
Note: Before you remove, please make sure you have the McAfee activation key handy, so that you can reinstall it after you check the issue.
https://service.mcafee.com/FAQDocument.aspx?id=TS101331
I would suggest you to remove all the third party antivirus applications using the following link.
https://support.microsoft.com/en-us/mats/program_install_and_uninstall?wa=wsignin1.0
While you reply with the status of the issue, please do let us know the results of SFC Scan performed.