how to retrieve the data by clicking the items in dropdownlist by using database in asp.net

1. Define SelectedIndexChanged Event for the DropDownList
2. Inside the Event, connect to Database, create a SqlCommand or SqlDataAdapter and provide the query you want to execute taking the DropDownList's SelectedValue
3. Execute the Command by ADO.NET methods like ExecuteReader, ExecuteNonQuery, ExecuteScalar or ExecuteXMLReader
4. Get the results and populate those on required controls

public void loadgrid()
{
      SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM [tablename] where [columnname] ='"+DropDownList1.SelecetedValue+"'", connStr); // Change query as per your requirement
            DataSet ds = new DataSet();
            da.Fill(ds);
            GridView1.DataSource = ds;
            GridView1.DataBind();
}
 

Set autopostback property of Dropdownlist to true and then call the above function  in DropDownList1_SelectedIndexChanged Event as follows

 protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        loadgrid();
    }