How did you attempt the upgrade? Through One-Click panel option, or via the WordPress dashboard?
If it was via One-Click, then it should have backed up your site into a .old folder you can rename via FTP. Try that first.
If it was through the WordPress dashboard, then you can roll back your site from the DreamHost panel under Domains -> Manage Domains -> Restore.
The database is usually left alone during an upgrade, but if it’s also blown up, can be restored from the DreamHost panel under Goodies -> MySQL Databases.