Responding to a hack
Apply Security Fixes applicable to your version
1. If any security fixes have been posted for your version, be sure you have installed them.
Site Audit to look for damaged or added files
2. You will need to do a full site audit to be sure all files are as you expect them to be. These steps can be used as a guide.
Secure Your Site Using the Security Recommendations
3. Ensure that you have secured your site following the recommendations provided with the code.
4. You may find these tips helpful: https://www.google.com/webmasters/hacked/