The redirect could be happening in a plugin or your active theme (or some a backdoor script injected somewhere in any file or folder on your site).
If you maintain backups, it’s probably easiest to switch to a pristine backup taken before all this started.
If you have don’t maintain backups(ahem!)… you need to manually search your WordPress files and folders AND database to find and remove the hack that is causing this redirect. (As you can’t access the dashboard, it’s not possible to use a plugin to automate this).
I would even backup my uploads folder locally, delete all the WordPress files and folders, re-install WordPress, plugins and theme, and restore my uploads folder (after thoroughly checking all folders to be sure there’s nothing hiding in there).
The FAQ I linked to earlier has links to 3rd-party articles on how to search for backdoor scripts and clean up a hacked WordPress site.