Webservice Webmethod not firing

Hi All.

I am using a dropdown list for states and a textbox for cities which have a Autocomplete extender to populate cities based on state selected in the dropdownlist.  from webservice method I am trying to call a function from datalayer. 

But its not working for me.

Please find below code

.aspx

<asp:DropDownList ID="ddlState" runat="server" AutoPostBack="true"
OnSelectedIndexChanged ="State_OnSelectedIndexChanged" style="width: 205px;"></asp:DropDownList>

asp:TextBox ID="txtCity" runat="server" style="width: 200px;"></asp:TextBox><asp:AutoCompleteExtender ID="aceShipCity" runat="server" TargetControlID="txtCity"
     CompletionInterval="100" CompletionSetCount="10" MinimumPrefixLength="1"
     EnableCaching="true" ServiceMethod="GetCities" UseContextKey="true"
     FirstRowSelected="false"></asp:AutoCompleteExtender>

.aspx.cs

protected void State_OnSelectedIndexChanged(object sender, EventArgs e)
{
try
{
aceCity.ContextKey = State.SelectedValue;


}
catch (Exception ex)
{
}

}

public static List<string> GetCities(string strPrefixText, string strStateContextKey)
    {
        List<string> lstCities = new List<string>();

        try
        {
            //get cities from the method 
            DataTable dtTempCities = new DataTable();
            CO objCO = new CO();
            dtTempCities = objCO.GetCities(strPrefixText, strStateContextKey);

            //store the datatable rows as list<string>
            if (dtTempCities.Rows.Count > 0)
            {
                for (int i = 0; i < dtTempCities.Rows.Count; i++)
                {
                    lstCities.Add(dtTempCities.Rows[i]["Cities"].ToString());
                }
            }

            else
            {
                //no records found
            }

        }

        catch (Exception ex)
        {
            //do nothing for now
        }

        return lstCities;
    }

Any help would be appreciated