I've seen problems with some major sites such as DAZ and Renderosity in recent days/weeks. As a rule I try a few times. If there are problems, then I wait an hour, or a few hours. The sites are usually back up again. Sometimes you can see evidence that someone is working on the site... Error messages might be different, you get different results each time you try... etc. That is usually good news because either a problem is being fixed or the site has some great new additions.