If you rightclick your site and select "view source" you will probably see that your labels id is not "lblSrNo". asp.net generates id's based on where in the control hierarchy they are. So you will have to either make sure asp.net tells your javascript the correct id, or use another jquery selector.
My suggestion is to add a css-class to all your lbl's and use classname instead of id:
$(document).ready(function(){
$('#btnDelete').live('click',function(){
var value = $(this).siblings('.YourClassName').text();
alert(value) ;
});
});