That would be because wordpress have your old domain in the configuration, which is stored in the database. Wordpress works in such way that it will try to redirect you to the domain in its configuration for administration.
There's two ways to sort this, one being changing back domain and editing the configuration and then change domain again. The problem with this is time.
The other option would be going in and editing the database using phpmyadmin. There's two entries you'll need to update in the table wp_options (where wp_ should be replaced with whatever prefix you have if you do not have the default one). The values to change is "home" and "siteurl".