You need to add parameter to the data adapter select command as below
da.SelectCommand.Parameters.AddWithValue("@searchkey", "%" + txt_Search.Text + "%");
Or you can use select command you already created
SqlDataAdapter adapter = new SqlDataAdapter(SqlCmd);
DataSet ds = new DataSet();
da.Fill(ds, "MLOANJOIN");
GridView1.DataSource =ds.Tables[0];
GridView1.DataBind();