Select2 doesn't show selected value

You need to use the initSelection option to set the initial value.

If you are using a pre-defined select element to create the select2, you can use the following method

$('select').select2().select2('val','3')

Demo: Fiddle

add a trigger change after setting val:

$('#my_id').val('3').trigger('change');

A very simple way to tackle this problem is :

//Step1: Here assuming id of your selectbox is my_id, so this will add selected attribute to 1st option
$('#my_id option').eq(0).prop('selected',true);

//Step2: Now reinitialize your select box

//This will work, if you haven't initialized selectbox
$('#my_id').select2(); 

or

//This will work, if you have initialized selectbox earlier, so destroy it first and initialise it
$('#my_id').select2('destroy').select2();