why drop down value is lost

[Deep]

hi friends,

1> I have one page which contains one dropdown. we can see data on
selection change event of dropdown.
2> when some one clicks data then it goes to login page using pop up.
user fills user id and password.
3> after filling right user id and password loginpage popup redirects
to myaccount page, then user close myaccount page and come again in
first page which is already open in background.
4> when he again change selection of drop down it shows that all
dropdown value has been finish so no data is showing and it shows
error.

I want to know why dropdown data is lost?
please help me

Thanks in advance