Sounds like the domain name changed nameservers before you backed up your data. If your old hosting account still exists, then you *should* be able to either look at the original email they sent you when you signed up, or contact the old host, to find out what the IP address is for the site so you can go in and retrieve your information. Alternatively, you could log in to wherever it is you manage your domain names (Verisign, Network Solutions, GoDaddy, etc.) and change the nameservers *back* to the old host. When they resolve (could take up to 24 hours) you can log back into your old hosting area as you would have before and back up your files and database. *Then* you’ll have to change the nameservers back to the new host, wait again for it to resolve, then upload your backups.
Bascially, a backup should have been done *before* you changed hosts. Hopefully you still have hosting time remaining on your old hosting account. If not, then you might get lucky and ask them for a backup before they delete your stuff (they’ll probably charge you for it), provided they haven’t done it yet. If they have, then yeah, everything’s gone.
but the codex has instruction on moving your wordpress information to a new server (aka switching hosts) here.
