I am not sure sorry. i just had a quick browse to the autotrader website and all seems well here.
do you get any errors displayed like 'page cannot be displayed' or similar. when you say the site dissappeared what do you see on the screen please?
do you have windows xp and use internet explorer 6 and service pack 2? if so some pop ups may be blocked on some sites.
do you see a yellow information bar just below the address bar? if you do then click this and choose 'temporarly allow pop ups' if the site works ok now and you can view the page you can always allow pop ups from this site by clicking 'tools/pop up blocker/pop up blocker settings' and add the site to your allowed list.
sorry if that doesnt help. can you give some more info please?