Populate a dropdownlist using another dropdownlist

What you need is called "cascading dropdownlist". Here's a few articles I found on the net:

ASP.NET WebForms: Creating Cascading DropDownLists in ASP.Net
ASP.NET with jQuery : Cascading DropDownLists with jQuery and ASP.NET
ASP.NET with AJAXControlToolkit: AJAX Cascading DropDownList sample with database using ASP.Net
ASP.NET MVC: Creating Cascading DropDownList In MVC Using Entity Framework And ADO.NET

Based on the links above, there are many ways to implement cascading dropdownlist. In the context of ASP.NET WebForms, you could either use jQuery/JavaScript AJAX, use the CascadingDropDown control from AJAXControlToolkit or manually doing it at the server. Here's a server-side quick implementation using the ADO.NET way:

 private string GetConnectionString()
 {
    //calling up the connection string that was set up from the web config file
     return System.Configuration.ConfigurationManager.ConnectionStrings["DBConnection"].ConnectionString;
 }

 private void BindDropDownList1()
 {
     DataTable dt = new DataTable();
     SqlConnection connection = new SqlConnection(GetConnectionString());
     try
     {
         connection.Open();
         string sqlStatement = "SELECT ColumnName * FROM TableName";
         SqlCommand sqlCmd = new SqlCommand(sqlStatement, connection);
         SqlDataAdapter sqlDa = new SqlDataAdapter(sqlCmd);
         sqlDa.Fill(dt);
         if (dt.Rows.Count > 0)
         {
             DropDownList1.DataSource =dt;
             DropDownList1.DataTextField = "ColumnName"; // the items to be displayed in the list items
             DropDownList1.DataValueField = "ColumnName"; // the id of the items displayed
             DropDownList1.DataBind();
         }
     }
     finally
     {
         connection.Close();
     }
 }

 private void BindDropDownList2(string field)
 {
     DataTable dt = new DataTable();
     SqlConnection connection = new SqlConnection(GetConnectionString());
     try
     {
         connection.Open();
         string sqlStatement = "SELECT * FROM Table WHERE ColumnName = @Value1";
         SqlCommand sqlCmd = new SqlCommand(sqlStatement, connection);
         sqlCmd .Parameters.AddWithValue("@Value1", field)
         SqlDataAdapter sqlDa = new SqlDataAdapter(sqlCmd);
         sqlDa.Fill(dt);
         if (dt.Rows.Count > 0)
         {
             DropDownList2.DataSource =dt;
             DropDownList2.DataTextField = "ColumnName"; // the items to be displayed in the list items
             DropDownList2.DataValueField = "ColumnName"; // the id of the items displayed
             DropDownList2.DataBind();
         }
     }
     finally
     {
         connection.Close();
     }
 }

protected void Page_Load(object sender, EventArgs e)
{
     if (!IsPostBack)
     {
         BindDropDownList1();
     }
 }

protected void DropDownList1_SelectedIndexChanged(object sender, System.EventArgs e)
{

   BindDropDownList2(DropDownList1.SelectedItem.Text);

}

The idea is to pass the selected item value when a selection is made from the dropdownlist and use that value to query your database and re-populate your second dropdownlist with the associated data.