Look at all of the ‘related’ links to the right of your question for help with a hacked site.
I would reinstall everything (WP, themes, plugins), check htaccess files (in all folders, not just the site root folder), change all site access passwords (ftp, host, email), check your local computer, look for unexpected files (in all folders, sort by date to help find anomalies).
If this is above your skillset, professional help may be required.
Added
For my own reference, I created a post about the things I do to fix a hacked site: http://securitydawg.com/recovering-from-a-hacked-wordpress-site/ .But the googles (and a search here) will give you much guidance on how to recover.