Hi modify your javascript function as "
Surname()
" by removing the
txt_
prefix.
Problem here is, your text box and the java script function has same name.
<asp:textbox id="txt_SurName" runat="server" onblur="SurName()" xmlns:asp="#unknown"></asp:textbox>
<script>
function SurName()
{
var x= document.getElementById("txt_SurName");
x.value=x.value.toUpperCase();
}
</script>