If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.

If that makes no difference, then it’s probably a much less pleasant situation. Carefully follow this guide. When you’re done, you may want to implement some (if not all) of the recommended security measures and start backing up your site.