Windows update is always failing with error code 0x80070643 and Defender stopped working after this

Hello imanojpatel, 

Please run Windows Update troubleshooter from Settings app > Update & security > Troubleshoot > Additional troubleshooters. See if it works.

If that doesn't work, try this:

Open administrative Command Prompt and type following commands one-by-one followed by Enter key.

net stop wuauserv

net stop cryptSvc

net stop bits

net stop msiserver

Ren C:\Windows\SoftwareDistribution SoftwareDistribution.old

Ren C:\Windows\System32\catroot2 Catroot2.old

net start wuauserv

net start cryptSvc

net start bits

net start msiserver

Close Command Prompt and see if Windows Updates works then.

Let us know if this helps!