Perhaps the following function would work,
$("#txtBoxClientId").change(function () {
if($(this).val() == "") {
$("#btnId").hide();
} else {
$("#btnId").show();
}
});
This would also let you make a condition to only show button when there are a few characters in the textbox.
The problem at that location can be answered in different ways,
1. The jQuery may not be loaded. If you open the External libraries tab, you will see you did not include anything.
2. JSFiddle writes all of the content in a nested HTML document, thus you cannot easily map the functions in there. Write a sample HTML document in your notepad, and then write the code as well. Run that file in any browser to get a good demo.