Which server is your site on?
My server ('Bixel') has had a couple of short outages over the last few days, but they are so short that by the time I get ready to lodge a support request, the server is back up.
Your best option is to use the facilities in the Support->Contact Support section of the Web Admin Panel.
As Simon said, if your domain is new, the problem might be related to DNS not having fully propagated across the Internet yet.
Save [color=#CC0000][b]$50[/b][/color] on DreamHost hosting using promo code [color=#CC0000][b]SAVEMONEY[/b][/color] ( Click for promo code details )