New admin users, hackers, registered and unable to interact with dashboard

Hello

I’m quite desperate…

I just discovered that 2 users from domain popdieta.pl have registered in the last days into my wordpress site, and besides of the hundreds of recent spams, i just saw that i’ve lost control of my wordpress installation, so when i enter into the dashboard, i can try to go to any place on the lateral menues, but the result is that i’m stuck in a blank page, which only shows:
the message that i can update wordpress to wordpress 5.2.2 and a strange message:
?Atención!
The file /www/doc/www.example.com/www/wp-content/advanced-cache.php already exists. Please manually delete it before using this plugin.

I replaced the website name for example.com for security reasons.

Does anybody knows how to recover my website instalation? it seems that i cannot even restore it to my last backup

Any help will be wellcomed, and also a way to avoid such attacks in the future.

The website apparently runs perfectly and besides from a warning mail from google search console, that now i see it’s fixed, everything else is fine.

The main question is how i can recover control of my wordpress dashboard
Best Regards
Jordi

The page I need help with: [log in to see the link]