If you have a dropdown like
<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged1" />
then you need a method in your code-behind with the exact name as it appears in the OnSelectedIndexChanged attribute
protected void DropDownList1_SelectedIndexChanged1(object sender, EventArgs e)
{
}
Note there is a "1" at the end of your function name so make sure the aspx and code-behind match exactly. Also make sure the method is "protected" or "public", not "private" or left default like
void DropDownList1_SelectedIndexChanged1(object sender, EventArgs e)
{
}