You see this:
The feature you are trying to use is on a network resource that is unavailable. This is happening because the cached installer is missing (in my case due to running a scan on folder sizes, seeing this particular windows folder with all the uninstallers (the cache) and removing them.)
I was experiencing an issue where my nuget packages would not restore the dlls. I would get Error CS0234 The type or namespace name 'Entity' does not exist in the namespace 'System.Data' (are you missing an assembly reference?) as the error but it would say all nuget packages are already installed.
I followed the sage advice of deleting everything in c:\users\\.nuget\packages as well as the packages folder in solution and force a restore.
I believe this could be applied to all package repositories in case this type of problems are faced.
By the way, check out Artifactory if your environment is hostile to public yum, apt-get, npm, nuget or any other repositories.