Had a frustrating problem with Windows 10 Defender over the weekend - it switched itself off and I could not enable it despite carrying out all of the suggested fixes. Eventually last night I did a check for windows updates and found that there was an update for Defender pending, but would not install. After power-up this evening I tried windows update again, this time it installed together with another Defender update and is now enabled itself again!

I had backed up all of my data intending to do a Windows 10 repair!