Advice on fixing a hacked web site

A while ago I took over support of a web site designed using Joomla! Last Friday the web site was hacked. When I contacted the ISP I was told "we don't support Joomla! sites". I tried doing a restore that failed and contacted the ISP again. This time he ran a scan on the web site for malware and gave me a list of files I should remove. As that list was pages long I gave up on that. Our President got email from sitelock.com telling him that the site was hacked and offering to help. Apparently that is a legitimate company.
As of now I haven't done anything else. Does anyone have any suggestions as to how should proceed?

Thanks

I have no experience with joomla but my general advice would be change all passwords associated with the site, joomla, database, especially ftp passwords. If you had a local copy of the clean joomla files on your computer you could do a file compare or look for files that shouldn't be there or with a recent modified date to identify hacked files then fix or replace them. Removing them if they're part of joomla would break functionality. But as you took the site over maybe you might not have a clean copy of the files locally. So then it might be better to do a fresh clean install of joomla and let it use the database of the old install and then delete the old install completely to make sure any hacked files are gone. No idea about sitelock but strange if you got an email out of the blue after the site is hacked, maybe they scan the web for hacked sites or the host directed them but still seems a bit questionable.

edit: also you would need to consider any theme files or legitimate joomla mods that may have been made to the site originally if you were going the clean install route.

I pay to have backups made on a daily, weekly and monthly basis. But doing a restore this time just did not work. As for sitelock, I asked the ISP and they said it is part of the package I have, so it is legitimate. My knowledge of Joomla! is minimal, so I really don't want to go there. The guy who built the website in the first place is currently not answering my emails.

Long time since I used Joomla Grace, but from the sound of it, as sitelock is part of your package it would do no harm contacting them.