HELP! My internal links are not working right now

I have had site visitors tell me that my internal links(from site to site) are not working and these are typically on public pages.

I have checked the link structure in all cases and it is correct. I get this message:

Sorry, we can't find the content you're looking for at this URL. Please try selecting a menu item from above or to the side of this message to get where you'd like to go.