selection frm drpdown the div becomes invisible

i have a div with a dropdown box with autopostback="true"
if i change the selction frm dropdown the div becomes invisible
which i dont want

how do i rectify it

at present i have a html button control with a function
to show the div

document.getElementById('DIV').style.display = 'block';

on load function i have
the div style=none

when i click on the button the div is visible
but when i change any selection frm drpdown the div becomes invisible ...