If you are seeing messages that it is missing or turned off, it’s usually because another antivirus program (like a pre-installed trial of McAfee or Norton) has disabled it to prevent conflicts. How to Enable Windows Defender on Windows 8 (64-bit)
If this fails with an error, it is usually because the Windows Update service itself is having issues. You can visit the Microsoft Update Catalog online, search for "Windows Defender Definition Update," and download the 64-bit specific executable file (mpam-fe.exe) to install the latest virus definitions manually. windows defender windows 8 download 64-bit