Your site has been hacked. Same advice I just posted in another thread:
Check your .htaccess file and restore it from a backup if you can.
You most likely have a 3rd party plugin or theme that is not secure.
there are 2 things to do here, fix the damage, but once that is done don’t stop, you also need to find and plug the hole or it will just be repeated.
Open a support ticket as well. Application specific issues are generally beyond the scope of support, however the may be able to provide you with excellent advice to follow.