I think our site was hacked

I tried logging in through my ISP and, when I get to the WordPress login page and type in my username and password, I get a message telling me I have an invalid username.

What’s the site?

You COULD get in to the back-back-end via PHPMyAdmin through your ISP probably, find the Administrator username, and change it (and the password) back to something you can use. Here’s a good tutorial on WP Beginner.

But, even if you are able to get in that way and restore your site backup, who knows where else the hackers might have gotten in and broken/messed with stuff?

I’d suggest just completely deleting the ENTIRE WordPress installation via FTP or your ISP and then totally reinstalling it all with a One-Click install or through your FTP. THEN, restore your backup when you are sure that you have a completely CLEAN version of WordPress reinstalled. (Or, alternatively, you could just download the latest WP distribution from www.ads-software.com and upload it via FTP and OVERWRITE any existing files.)

And change your password to something nice and strong. And be sure NOT to use the default “admin” as your Administrator username (it’s easy for hackers to target that username first because it is/was the default for so long).

Here’s a nice post about good passwords.
Here’s some more suggestions to strengthen your WordPress admin.

You need to start working your way through these resources:
https://codex.www.ads-software.com/FAQ_My_site_was_hacked
https://www.ads-software.com/support/topic/268083#post-1065779
https://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
https://ottopress.com/2009/hacked-wordpress-backdoors/

Additional Resources:
Hardening WordPress
https://sitecheck.sucuri.net/scanner/
https://www.unmaskparasites.com/
https://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html
https://blog.sucuri.net/2010/07/understanding-and-cleaning-the-pharma-hack-on-wordpress.html

Excellent resources posted above ^ – have worked through many of those myself ^^;

I particularly appreciate Sucuri: https://sitecheck.sucuri.net/scanner/