无法找到所请求的。net Framework数据提供程序。它可能不会被安装。(System.Data.SQLClient)

I tried to look at the data connection between my visual studio and SQL server, however it keeps on saying "Unable to find the requested .Net Framework Data Provider. It may not be installed." How can I get this fixed please? It was working before, suddenly it stopped working. I checked in the C:/windows/assembly System.Data.SQLClient is not there. I tried to look at the data connection between