here is the simple steps to reinstall cortana and all built-in windows 10 app :
if you just want to disable cortana please follow : disable cortana windows 10
if you just want to turn on cortana please follow : turn on cortana windows 10
Open powershell by click windows startmenu and type “powershell”. Then righclick on windows powershell, Run as administrator.
copy and paste this code to your powershell :