This page carries a fixit for Defender although for a slightly different issue. Try running either it or following the suggestions:when I tried to run defender I get the error that MsMpRes.dll is missing, so it will not run. I went to all of the dll download sites and that dll was not on any of the sites. I am running sfc /scannow and letting it run overnight.
https://support.microsoft.com/en-us/kb/931849