SITE HACKED: Possibly from contact page message, front-end gone missing

My site was hacked today, and my best guess is that it relates to two consecutive messages received from my contact page (WP form), which seemed suspiciously like junk mail; a few minutes later I received a similar email to another address listed on that page. All were from james wise [ redacted ].

All of my content and settings are still on the dashboard backend, untouched, but the front end of my site went almost entirely missing. The only things that were left were the header, primary navigation (those links still work and display properly once redirected), and the most recent article posted. I backed up all the content, increased security, changed my username and password, and put up a landing page explaining that the site is temporarily down; but I have no idea how to fix the problem with the home page.

I’m assuming the template files were changed in some way, but I’m a total amateur as far as working with the files or adding any custom code (I’ve worked almost entirely with developers), and need some guidance about how to proceed. Any help would be greatly appreciated!