You need to look at all files via your hosting File Manager (or FTP). Look for any code that shouldn’t be there.
See my answer to Coinhive Malware on WordPress websites , of things to check.
And here’s the process I use to clean up hacked WP sites: https://securitydawg.com/recovering-from-a-hacked-wordpress-site/ . Basically: change credentials on everything; look at all file contents for hacked code; reinstall everything from known good/original source; reinstall latest WP; and more.