In order to reinstall Windows Defender, you need to re-install your operating system or to perform a Windows repair. Also, you can reset it by turning it off and then on. Go to Start, type Windows Defender and open the program. Click the Tools menu, select Options and then press Administrator. Un-check the Use this program box and click Save. After that, reboot your computer, type again Windows Defender in the Search programs and files field from the Start menu, hit the Enter key and press click here to turn it on.
Note: If you don't have Windows Defender on your computer or if you don't want to re-install/repair the operating system, you can download and install the offline version from Microsoft Download Center.