Hi,
your not assign the DataSet to your DropDownList.
Please try this
SqlCommand cmd = new SqlCommand("select * from user_login where groupid=1", cnn);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
ddlstudentname.DataSource=ds;
ddlstudentname.DataTextField="name";
ddlstudentname.DataVlaueField="name";
ddlstudentname.DataBind();