Great question — answering as the UphillTech team:
Although hacked sites are not within scope here (and your question will be closed because of that policy), there are questions here about hacked sites that have answers that will be helpful (I've written a couple of them). Cleanup is time-consuming, but possible. Generally - Change passwords on everything, including your admin-level user and hosting and FTP and databases. Manually reinstall WP Core, themes, and plugins. Delete any unused themes and plugins. Look at all files for added code (like in your htaccess file). Look for htaccess files in other folders. Look at contents of all ICO files (malware can be in a fake ICO file). Remove any improper admin-level accounts. Make a new...
Full answer by Rick Hellewell: https://wordpress.stackexchange.com/a/416252 (CC BY-SA 4.0).
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