It's not clear by what you mean you deleted the folder "Resources" as all the items in the horizontal navigation represent a page on your website, and if you delete a page it gets deleted from the Navigation i.e., in your case the Horizontal navigation bar.

Just checked your site and indeed it is a broken link, Is the Horizontal navigation on this site an auto-nav block or is it a hard-coded html navigation in your site theme.

The site was designed before my time, and somehow no one on staff seems to know the answer to this, but based on what I've tried, it appears that the horizontal nav is hard-coded html in the theme.

If that is the case then it is a wrong approach, as Concrete 5 has an extensive navigation block known as auto-nav which comes out of the box with concrete5, As the name suggests you just need to read about how theme creation in concrete 5 works, you could check the comprehensive Documentation and how-to section, once you understand how themes work you need to create an editable area in place of the hard-coded html navigation, so that you could add the auto-nav block instead also try reading about how auto-nav works.

If it broke, I can bet it's not C5. If you already cleared the cache, you most likely (I'm 99.99% sure) have to edit the HTML. The browser renders a simple table. You just have to edit it. Check what you have to Edit:

-If it is in the Default for the page (Probably is), open:
Dashboard > Pages and Themes > Page Types (On the top of the section).
-If it is in the Theme itself, you have to Edit the theme. Just find the /themes/(theme name) folder, and check the php files that have the menu. As you find it, just edit it.

If it is HTML, PHP, JS or what, we can only be sure looking at the files themselves. Probably HTML, no one does this much PHP to render something that breaks this easily, though it can be done.

If you need any more "technical" help, just post where you got with this info. It's probably quite easy. :D

Edit: Also, check the navigation as Pritam said. Don't stop with the quick-fix, it will still be "breakable", it's best to have something that works. A bit more work now, but makes life easier in the future.

It's hard coded in. Each link is a jpg image that links to a specified url.
Quickest/dirtiest fix may be adding a beneficial link/page name by creating new jpeg sized to replace the original(links, contact, map, whatever) and either creating a new page to match the menu or even linking to one of the other pages temporarily.
Sorta halfway done, but will buy you time while considering if you want to replace the entire nav section

Might be easier if he just deleted the section and centered the whole table. The images are pretty simple, no need to "fill" the gap, just center it for a quick-fix, and then make (Or at least consider making) it work properly with Auto-Nav.

CSS nowadays does a job even prettier than the images on the menu, anyways.