Sorry, but I'm not going to a site that's been hacked. Too easy to get some nasty malware.
Find all the applications that are running on the site and check for security issues at
Secunia.com. Blogs, chat applications, some content management systems, etc. often have vulnerabilities that are exploited over and over until they are upgraded.
If there is a custom application on the site, it may have security issues. They must be addressed. An audit is available at
PHP Security Consortium.
It is also possible that there is a file on your server that is allowing someone else to upload code. Until you remove it, they will control the server.
I'm assuming it an HTML site on shared hosting, you have one account on a publicly accessible server. In that case, my recommendation would be to download ALL the files from the www or public_html account (or whichever directory index.html is in) and all the subdirectories. Then, look at them on your PC. If there are any files you didn't put there, delete them. Another approach would be to delete everything and put a clean copy up.
You could also contact the hosting company for help. They will be reluctant to delete any files, because they aren't familiar with your site - but they also want to protect their server.